1224 lines
33 KiB
C
1224 lines
33 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/* kernel/rwsem.c: R/W semaphores, public implementation
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*
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* Written by David Howells (dhowells@redhat.com).
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* Derived from asm-i386/semaphore.h
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*
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* Writer lock-stealing by Alex Shi <alex.shi@intel.com>
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* and Michel Lespinasse <walken@google.com>
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*
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* Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
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* and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
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*
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* Rwsem count bit fields re-definition and rwsem rearchitecture by
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* Waiman Long <longman@redhat.com> and
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* Peter Zijlstra <peterz@infradead.org>.
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*/
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#include <linux/types.h>
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#include <linux/kernel.h>
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#include <linux/sched.h>
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#include <linux/sched/rt.h>
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#include <linux/sched/task.h>
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#include <linux/sched/debug.h>
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#include <linux/sched/wake_q.h>
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#include <linux/sched/signal.h>
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#include <linux/export.h>
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#include <linux/rwsem.h>
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#include <linux/atomic.h>
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#include "rwsem.h"
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#include "lock_events.h"
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/*
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* The least significant 2 bits of the owner value has the following
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* meanings when set.
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* - RWSEM_READER_OWNED (bit 0): The rwsem is owned by readers
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* - RWSEM_ANONYMOUSLY_OWNED (bit 1): The rwsem is anonymously owned,
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* i.e. the owner(s) cannot be readily determined. It can be reader
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* owned or the owning writer is indeterminate.
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*
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* When a writer acquires a rwsem, it puts its task_struct pointer
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* into the owner field. It is cleared after an unlock.
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*
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* When a reader acquires a rwsem, it will also puts its task_struct
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* pointer into the owner field with both the RWSEM_READER_OWNED and
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* RWSEM_ANONYMOUSLY_OWNED bits set. On unlock, the owner field will
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* largely be left untouched. So for a free or reader-owned rwsem,
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* the owner value may contain information about the last reader that
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* acquires the rwsem. The anonymous bit is set because that particular
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* reader may or may not still own the lock.
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*
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* That information may be helpful in debugging cases where the system
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* seems to hang on a reader owned rwsem especially if only one reader
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* is involved. Ideally we would like to track all the readers that own
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* a rwsem, but the overhead is simply too big.
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*/
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#define RWSEM_READER_OWNED (1UL << 0)
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#define RWSEM_ANONYMOUSLY_OWNED (1UL << 1)
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#ifdef CONFIG_DEBUG_RWSEMS
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# define DEBUG_RWSEMS_WARN_ON(c, sem) do { \
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if (!debug_locks_silent && \
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WARN_ONCE(c, "DEBUG_RWSEMS_WARN_ON(%s): count = 0x%lx, owner = 0x%lx, curr 0x%lx, list %sempty\n",\
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#c, atomic_long_read(&(sem)->count), \
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(long)((sem)->owner), (long)current, \
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list_empty(&(sem)->wait_list) ? "" : "not ")) \
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debug_locks_off(); \
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} while (0)
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#else
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# define DEBUG_RWSEMS_WARN_ON(c, sem)
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#endif
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/*
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* The definition of the atomic counter in the semaphore:
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*
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* Bit 0 - writer locked bit
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* Bit 1 - waiters present bit
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* Bit 2 - lock handoff bit
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* Bits 3-7 - reserved
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* Bits 8-X - 24-bit (32-bit) or 56-bit reader count
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*
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* atomic_long_fetch_add() is used to obtain reader lock, whereas
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* atomic_long_cmpxchg() will be used to obtain writer lock.
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*
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* There are three places where the lock handoff bit may be set or cleared.
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* 1) rwsem_mark_wake() for readers.
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* 2) rwsem_try_write_lock() for writers.
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* 3) Error path of rwsem_down_write_slowpath().
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*
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* For all the above cases, wait_lock will be held. A writer must also
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* be the first one in the wait_list to be eligible for setting the handoff
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* bit. So concurrent setting/clearing of handoff bit is not possible.
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*/
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#define RWSEM_WRITER_LOCKED (1UL << 0)
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#define RWSEM_FLAG_WAITERS (1UL << 1)
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#define RWSEM_FLAG_HANDOFF (1UL << 2)
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#define RWSEM_READER_SHIFT 8
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#define RWSEM_READER_BIAS (1UL << RWSEM_READER_SHIFT)
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#define RWSEM_READER_MASK (~(RWSEM_READER_BIAS - 1))
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#define RWSEM_WRITER_MASK RWSEM_WRITER_LOCKED
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#define RWSEM_LOCK_MASK (RWSEM_WRITER_MASK|RWSEM_READER_MASK)
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#define RWSEM_READ_FAILED_MASK (RWSEM_WRITER_MASK|RWSEM_FLAG_WAITERS|\
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RWSEM_FLAG_HANDOFF)
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/*
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* All writes to owner are protected by WRITE_ONCE() to make sure that
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* store tearing can't happen as optimistic spinners may read and use
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* the owner value concurrently without lock. Read from owner, however,
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* may not need READ_ONCE() as long as the pointer value is only used
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* for comparison and isn't being dereferenced.
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*/
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static inline void rwsem_set_owner(struct rw_semaphore *sem)
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{
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WRITE_ONCE(sem->owner, current);
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}
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static inline void rwsem_clear_owner(struct rw_semaphore *sem)
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{
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WRITE_ONCE(sem->owner, NULL);
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}
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/*
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* The task_struct pointer of the last owning reader will be left in
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* the owner field.
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*
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* Note that the owner value just indicates the task has owned the rwsem
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* previously, it may not be the real owner or one of the real owners
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* anymore when that field is examined, so take it with a grain of salt.
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*/
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static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem,
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struct task_struct *owner)
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{
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unsigned long val = (unsigned long)owner | RWSEM_READER_OWNED
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| RWSEM_ANONYMOUSLY_OWNED;
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WRITE_ONCE(sem->owner, (struct task_struct *)val);
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}
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static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
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{
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__rwsem_set_reader_owned(sem, current);
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}
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/*
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* Return true if the a rwsem waiter can spin on the rwsem's owner
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* and steal the lock, i.e. the lock is not anonymously owned.
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* N.B. !owner is considered spinnable.
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*/
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static inline bool is_rwsem_owner_spinnable(struct task_struct *owner)
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{
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return !((unsigned long)owner & RWSEM_ANONYMOUSLY_OWNED);
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}
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/*
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* Return true if rwsem is owned by an anonymous writer or readers.
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*/
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static inline bool rwsem_has_anonymous_owner(struct task_struct *owner)
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{
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return (unsigned long)owner & RWSEM_ANONYMOUSLY_OWNED;
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}
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#ifdef CONFIG_DEBUG_RWSEMS
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/*
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* With CONFIG_DEBUG_RWSEMS configured, it will make sure that if there
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* is a task pointer in owner of a reader-owned rwsem, it will be the
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* real owner or one of the real owners. The only exception is when the
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* unlock is done by up_read_non_owner().
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*/
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static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
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{
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unsigned long val = (unsigned long)current | RWSEM_READER_OWNED
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| RWSEM_ANONYMOUSLY_OWNED;
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if (READ_ONCE(sem->owner) == (struct task_struct *)val)
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cmpxchg_relaxed((unsigned long *)&sem->owner, val,
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RWSEM_READER_OWNED | RWSEM_ANONYMOUSLY_OWNED);
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}
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#else
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static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
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{
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}
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#endif
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/*
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* Guide to the rw_semaphore's count field.
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*
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* When the RWSEM_WRITER_LOCKED bit in count is set, the lock is owned
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* by a writer.
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*
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* The lock is owned by readers when
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* (1) the RWSEM_WRITER_LOCKED isn't set in count,
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* (2) some of the reader bits are set in count, and
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* (3) the owner field has RWSEM_READ_OWNED bit set.
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*
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* Having some reader bits set is not enough to guarantee a readers owned
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* lock as the readers may be in the process of backing out from the count
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* and a writer has just released the lock. So another writer may steal
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* the lock immediately after that.
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*/
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/*
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* Initialize an rwsem:
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*/
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void __init_rwsem(struct rw_semaphore *sem, const char *name,
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struct lock_class_key *key)
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{
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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/*
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* Make sure we are not reinitializing a held semaphore:
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*/
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debug_check_no_locks_freed((void *)sem, sizeof(*sem));
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lockdep_init_map(&sem->dep_map, name, key, 0);
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#endif
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atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
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raw_spin_lock_init(&sem->wait_lock);
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INIT_LIST_HEAD(&sem->wait_list);
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sem->owner = NULL;
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#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
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osq_lock_init(&sem->osq);
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#endif
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}
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EXPORT_SYMBOL(__init_rwsem);
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enum rwsem_waiter_type {
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RWSEM_WAITING_FOR_WRITE,
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RWSEM_WAITING_FOR_READ
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};
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struct rwsem_waiter {
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struct list_head list;
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struct task_struct *task;
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enum rwsem_waiter_type type;
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unsigned long timeout;
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};
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#define rwsem_first_waiter(sem) \
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list_first_entry(&sem->wait_list, struct rwsem_waiter, list)
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enum rwsem_wake_type {
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RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */
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RWSEM_WAKE_READERS, /* Wake readers only */
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RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */
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};
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enum writer_wait_state {
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WRITER_NOT_FIRST, /* Writer is not first in wait list */
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WRITER_FIRST, /* Writer is first in wait list */
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WRITER_HANDOFF /* Writer is first & handoff needed */
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};
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/*
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* The typical HZ value is either 250 or 1000. So set the minimum waiting
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* time to at least 4ms or 1 jiffy (if it is higher than 4ms) in the wait
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* queue before initiating the handoff protocol.
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*/
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#define RWSEM_WAIT_TIMEOUT DIV_ROUND_UP(HZ, 250)
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/*
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* handle the lock release when processes blocked on it that can now run
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* - if we come here from up_xxxx(), then the RWSEM_FLAG_WAITERS bit must
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* have been set.
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* - there must be someone on the queue
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* - the wait_lock must be held by the caller
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* - tasks are marked for wakeup, the caller must later invoke wake_up_q()
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* to actually wakeup the blocked task(s) and drop the reference count,
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* preferably when the wait_lock is released
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* - woken process blocks are discarded from the list after having task zeroed
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* - writers are only marked woken if downgrading is false
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*/
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static void rwsem_mark_wake(struct rw_semaphore *sem,
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enum rwsem_wake_type wake_type,
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struct wake_q_head *wake_q)
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{
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struct rwsem_waiter *waiter, *tmp;
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long oldcount, woken = 0, adjustment = 0;
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struct list_head wlist;
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lockdep_assert_held(&sem->wait_lock);
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/*
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* Take a peek at the queue head waiter such that we can determine
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* the wakeup(s) to perform.
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*/
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waiter = rwsem_first_waiter(sem);
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if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
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if (wake_type == RWSEM_WAKE_ANY) {
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/*
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* Mark writer at the front of the queue for wakeup.
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* Until the task is actually later awoken later by
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* the caller, other writers are able to steal it.
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* Readers, on the other hand, will block as they
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* will notice the queued writer.
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*/
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wake_q_add(wake_q, waiter->task);
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lockevent_inc(rwsem_wake_writer);
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}
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return;
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}
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/*
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* Writers might steal the lock before we grant it to the next reader.
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* We prefer to do the first reader grant before counting readers
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* so we can bail out early if a writer stole the lock.
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*/
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if (wake_type != RWSEM_WAKE_READ_OWNED) {
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adjustment = RWSEM_READER_BIAS;
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oldcount = atomic_long_fetch_add(adjustment, &sem->count);
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if (unlikely(oldcount & RWSEM_WRITER_MASK)) {
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/*
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* When we've been waiting "too" long (for writers
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* to give up the lock), request a HANDOFF to
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* force the issue.
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*/
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if (!(oldcount & RWSEM_FLAG_HANDOFF) &&
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time_after(jiffies, waiter->timeout)) {
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adjustment -= RWSEM_FLAG_HANDOFF;
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lockevent_inc(rwsem_rlock_handoff);
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}
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atomic_long_add(-adjustment, &sem->count);
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return;
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}
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/*
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* Set it to reader-owned to give spinners an early
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* indication that readers now have the lock.
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*/
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__rwsem_set_reader_owned(sem, waiter->task);
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}
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/*
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* Grant an infinite number of read locks to the readers at the front
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* of the queue. We know that woken will be at least 1 as we accounted
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* for above. Note we increment the 'active part' of the count by the
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* number of readers before waking any processes up.
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*
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* We have to do wakeup in 2 passes to prevent the possibility that
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* the reader count may be decremented before it is incremented. It
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* is because the to-be-woken waiter may not have slept yet. So it
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* may see waiter->task got cleared, finish its critical section and
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* do an unlock before the reader count increment.
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*
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* 1) Collect the read-waiters in a separate list, count them and
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* fully increment the reader count in rwsem.
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* 2) For each waiters in the new list, clear waiter->task and
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* put them into wake_q to be woken up later.
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*/
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list_for_each_entry(waiter, &sem->wait_list, list) {
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if (waiter->type == RWSEM_WAITING_FOR_WRITE)
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break;
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woken++;
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}
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list_cut_before(&wlist, &sem->wait_list, &waiter->list);
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adjustment = woken * RWSEM_READER_BIAS - adjustment;
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lockevent_cond_inc(rwsem_wake_reader, woken);
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if (list_empty(&sem->wait_list)) {
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/* hit end of list above */
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adjustment -= RWSEM_FLAG_WAITERS;
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}
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/*
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* When we've woken a reader, we no longer need to force writers
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* to give up the lock and we can clear HANDOFF.
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*/
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if (woken && (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF))
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adjustment -= RWSEM_FLAG_HANDOFF;
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if (adjustment)
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atomic_long_add(adjustment, &sem->count);
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/* 2nd pass */
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list_for_each_entry_safe(waiter, tmp, &wlist, list) {
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struct task_struct *tsk;
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tsk = waiter->task;
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get_task_struct(tsk);
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/*
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* Ensure calling get_task_struct() before setting the reader
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* waiter to nil such that rwsem_down_read_slowpath() cannot
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* race with do_exit() by always holding a reference count
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* to the task to wakeup.
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*/
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smp_store_release(&waiter->task, NULL);
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/*
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* Ensure issuing the wakeup (either by us or someone else)
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* after setting the reader waiter to nil.
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*/
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wake_q_add_safe(wake_q, tsk);
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}
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}
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/*
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* This function must be called with the sem->wait_lock held to prevent
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* race conditions between checking the rwsem wait list and setting the
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* sem->count accordingly.
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*
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* If wstate is WRITER_HANDOFF, it will make sure that either the handoff
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* bit is set or the lock is acquired with handoff bit cleared.
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*/
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static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
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enum writer_wait_state wstate)
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{
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long count, new;
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lockdep_assert_held(&sem->wait_lock);
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count = atomic_long_read(&sem->count);
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do {
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bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
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if (has_handoff && wstate == WRITER_NOT_FIRST)
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return false;
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new = count;
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if (count & RWSEM_LOCK_MASK) {
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if (has_handoff || (wstate != WRITER_HANDOFF))
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return false;
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new |= RWSEM_FLAG_HANDOFF;
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} else {
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new |= RWSEM_WRITER_LOCKED;
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new &= ~RWSEM_FLAG_HANDOFF;
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if (list_is_singular(&sem->wait_list))
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new &= ~RWSEM_FLAG_WAITERS;
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}
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} while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));
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/*
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* We have either acquired the lock with handoff bit cleared or
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* set the handoff bit.
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*/
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if (new & RWSEM_FLAG_HANDOFF)
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return false;
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rwsem_set_owner(sem);
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return true;
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}
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#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
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/*
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* Try to acquire write lock before the writer has been put on wait queue.
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*/
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static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
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{
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long count = atomic_long_read(&sem->count);
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while (!(count & (RWSEM_LOCK_MASK|RWSEM_FLAG_HANDOFF))) {
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if (atomic_long_try_cmpxchg_acquire(&sem->count, &count,
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count | RWSEM_WRITER_LOCKED)) {
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rwsem_set_owner(sem);
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lockevent_inc(rwsem_opt_wlock);
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return true;
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}
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}
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return false;
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}
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static inline bool owner_on_cpu(struct task_struct *owner)
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{
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/*
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* As lock holder preemption issue, we both skip spinning if
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* task is not on cpu or its cpu is preempted
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*/
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return owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
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}
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|
static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
|
|
{
|
|
struct task_struct *owner;
|
|
bool ret = true;
|
|
|
|
BUILD_BUG_ON(!rwsem_has_anonymous_owner(RWSEM_OWNER_UNKNOWN));
|
|
|
|
if (need_resched())
|
|
return false;
|
|
|
|
rcu_read_lock();
|
|
owner = READ_ONCE(sem->owner);
|
|
if (owner) {
|
|
ret = is_rwsem_owner_spinnable(owner) &&
|
|
owner_on_cpu(owner);
|
|
}
|
|
rcu_read_unlock();
|
|
return ret;
|
|
}
|
|
|
|
/*
|
|
* The rwsem_spin_on_owner() function returns the folowing 4 values
|
|
* depending on the lock owner state.
|
|
* OWNER_NULL : owner is currently NULL
|
|
* OWNER_WRITER: when owner changes and is a writer
|
|
* OWNER_READER: when owner changes and the new owner may be a reader.
|
|
* OWNER_NONSPINNABLE:
|
|
* when optimistic spinning has to stop because either the
|
|
* owner stops running, is unknown, or its timeslice has
|
|
* been used up.
|
|
*/
|
|
enum owner_state {
|
|
OWNER_NULL = 1 << 0,
|
|
OWNER_WRITER = 1 << 1,
|
|
OWNER_READER = 1 << 2,
|
|
OWNER_NONSPINNABLE = 1 << 3,
|
|
};
|
|
#define OWNER_SPINNABLE (OWNER_NULL | OWNER_WRITER)
|
|
|
|
static inline enum owner_state rwsem_owner_state(unsigned long owner)
|
|
{
|
|
if (!owner)
|
|
return OWNER_NULL;
|
|
|
|
if (owner & RWSEM_ANONYMOUSLY_OWNED)
|
|
return OWNER_NONSPINNABLE;
|
|
|
|
if (owner & RWSEM_READER_OWNED)
|
|
return OWNER_READER;
|
|
|
|
return OWNER_WRITER;
|
|
}
|
|
|
|
static noinline enum owner_state rwsem_spin_on_owner(struct rw_semaphore *sem)
|
|
{
|
|
struct task_struct *tmp, *owner = READ_ONCE(sem->owner);
|
|
enum owner_state state = rwsem_owner_state((unsigned long)owner);
|
|
|
|
if (state != OWNER_WRITER)
|
|
return state;
|
|
|
|
rcu_read_lock();
|
|
for (;;) {
|
|
if (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF) {
|
|
state = OWNER_NONSPINNABLE;
|
|
break;
|
|
}
|
|
|
|
tmp = READ_ONCE(sem->owner);
|
|
if (tmp != owner) {
|
|
state = rwsem_owner_state((unsigned long)tmp);
|
|
break;
|
|
}
|
|
|
|
/*
|
|
* Ensure we emit the owner->on_cpu, dereference _after_
|
|
* checking sem->owner still matches owner, if that fails,
|
|
* owner might point to free()d memory, if it still matches,
|
|
* the rcu_read_lock() ensures the memory stays valid.
|
|
*/
|
|
barrier();
|
|
|
|
if (need_resched() || !owner_on_cpu(owner)) {
|
|
state = OWNER_NONSPINNABLE;
|
|
break;
|
|
}
|
|
|
|
cpu_relax();
|
|
}
|
|
rcu_read_unlock();
|
|
|
|
return state;
|
|
}
|
|
|
|
static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
|
|
{
|
|
bool taken = false;
|
|
int prev_owner_state = OWNER_NULL;
|
|
|
|
preempt_disable();
|
|
|
|
/* sem->wait_lock should not be held when doing optimistic spinning */
|
|
if (!rwsem_can_spin_on_owner(sem))
|
|
goto done;
|
|
|
|
if (!osq_lock(&sem->osq))
|
|
goto done;
|
|
|
|
/*
|
|
* Optimistically spin on the owner field and attempt to acquire the
|
|
* lock whenever the owner changes. Spinning will be stopped when:
|
|
* 1) the owning writer isn't running; or
|
|
* 2) readers own the lock as we can't determine if they are
|
|
* actively running or not.
|
|
*/
|
|
for (;;) {
|
|
enum owner_state owner_state = rwsem_spin_on_owner(sem);
|
|
|
|
if (!(owner_state & OWNER_SPINNABLE))
|
|
break;
|
|
|
|
/*
|
|
* Try to acquire the lock
|
|
*/
|
|
if (rwsem_try_write_lock_unqueued(sem)) {
|
|
taken = true;
|
|
break;
|
|
}
|
|
|
|
/*
|
|
* An RT task cannot do optimistic spinning if it cannot
|
|
* be sure the lock holder is running or live-lock may
|
|
* happen if the current task and the lock holder happen
|
|
* to run in the same CPU. However, aborting optimistic
|
|
* spinning while a NULL owner is detected may miss some
|
|
* opportunity where spinning can continue without causing
|
|
* problem.
|
|
*
|
|
* There are 2 possible cases where an RT task may be able
|
|
* to continue spinning.
|
|
*
|
|
* 1) The lock owner is in the process of releasing the
|
|
* lock, sem->owner is cleared but the lock has not
|
|
* been released yet.
|
|
* 2) The lock was free and owner cleared, but another
|
|
* task just comes in and acquire the lock before
|
|
* we try to get it. The new owner may be a spinnable
|
|
* writer.
|
|
*
|
|
* To take advantage of two scenarios listed agove, the RT
|
|
* task is made to retry one more time to see if it can
|
|
* acquire the lock or continue spinning on the new owning
|
|
* writer. Of course, if the time lag is long enough or the
|
|
* new owner is not a writer or spinnable, the RT task will
|
|
* quit spinning.
|
|
*
|
|
* If the owner is a writer, the need_resched() check is
|
|
* done inside rwsem_spin_on_owner(). If the owner is not
|
|
* a writer, need_resched() check needs to be done here.
|
|
*/
|
|
if (owner_state != OWNER_WRITER) {
|
|
if (need_resched())
|
|
break;
|
|
if (rt_task(current) &&
|
|
(prev_owner_state != OWNER_WRITER))
|
|
break;
|
|
}
|
|
prev_owner_state = owner_state;
|
|
|
|
/*
|
|
* The cpu_relax() call is a compiler barrier which forces
|
|
* everything in this loop to be re-loaded. We don't need
|
|
* memory barriers as we'll eventually observe the right
|
|
* values at the cost of a few extra spins.
|
|
*/
|
|
cpu_relax();
|
|
}
|
|
osq_unlock(&sem->osq);
|
|
done:
|
|
preempt_enable();
|
|
lockevent_cond_inc(rwsem_opt_fail, !taken);
|
|
return taken;
|
|
}
|
|
#else
|
|
static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
|
|
{
|
|
return false;
|
|
}
|
|
#endif
|
|
|
|
/*
|
|
* Wait for the read lock to be granted
|
|
*/
|
|
static struct rw_semaphore __sched *
|
|
rwsem_down_read_slowpath(struct rw_semaphore *sem, int state)
|
|
{
|
|
long count, adjustment = -RWSEM_READER_BIAS;
|
|
struct rwsem_waiter waiter;
|
|
DEFINE_WAKE_Q(wake_q);
|
|
|
|
waiter.task = current;
|
|
waiter.type = RWSEM_WAITING_FOR_READ;
|
|
waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
|
|
|
|
raw_spin_lock_irq(&sem->wait_lock);
|
|
if (list_empty(&sem->wait_list)) {
|
|
/*
|
|
* In case the wait queue is empty and the lock isn't owned
|
|
* by a writer or has the handoff bit set, this reader can
|
|
* exit the slowpath and return immediately as its
|
|
* RWSEM_READER_BIAS has already been set in the count.
|
|
*/
|
|
if (!(atomic_long_read(&sem->count) &
|
|
(RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
rwsem_set_reader_owned(sem);
|
|
lockevent_inc(rwsem_rlock_fast);
|
|
return sem;
|
|
}
|
|
adjustment += RWSEM_FLAG_WAITERS;
|
|
}
|
|
list_add_tail(&waiter.list, &sem->wait_list);
|
|
|
|
/* we're now waiting on the lock, but no longer actively locking */
|
|
count = atomic_long_add_return(adjustment, &sem->count);
|
|
|
|
/*
|
|
* If there are no active locks, wake the front queued process(es).
|
|
*
|
|
* If there are no writers and we are first in the queue,
|
|
* wake our own waiter to join the existing active readers !
|
|
*/
|
|
if (!(count & RWSEM_LOCK_MASK) ||
|
|
(!(count & RWSEM_WRITER_MASK) && (adjustment & RWSEM_FLAG_WAITERS)))
|
|
rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
|
|
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
wake_up_q(&wake_q);
|
|
|
|
/* wait to be given the lock */
|
|
while (true) {
|
|
set_current_state(state);
|
|
if (!waiter.task)
|
|
break;
|
|
if (signal_pending_state(state, current)) {
|
|
raw_spin_lock_irq(&sem->wait_lock);
|
|
if (waiter.task)
|
|
goto out_nolock;
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
break;
|
|
}
|
|
schedule();
|
|
lockevent_inc(rwsem_sleep_reader);
|
|
}
|
|
|
|
__set_current_state(TASK_RUNNING);
|
|
lockevent_inc(rwsem_rlock);
|
|
return sem;
|
|
out_nolock:
|
|
list_del(&waiter.list);
|
|
if (list_empty(&sem->wait_list)) {
|
|
atomic_long_andnot(RWSEM_FLAG_WAITERS|RWSEM_FLAG_HANDOFF,
|
|
&sem->count);
|
|
}
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
__set_current_state(TASK_RUNNING);
|
|
lockevent_inc(rwsem_rlock_fail);
|
|
return ERR_PTR(-EINTR);
|
|
}
|
|
|
|
/*
|
|
* Wait until we successfully acquire the write lock
|
|
*/
|
|
static struct rw_semaphore *
|
|
rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
|
|
{
|
|
long count;
|
|
enum writer_wait_state wstate;
|
|
struct rwsem_waiter waiter;
|
|
struct rw_semaphore *ret = sem;
|
|
DEFINE_WAKE_Q(wake_q);
|
|
|
|
/* do optimistic spinning and steal lock if possible */
|
|
if (rwsem_optimistic_spin(sem))
|
|
return sem;
|
|
|
|
/*
|
|
* Optimistic spinning failed, proceed to the slowpath
|
|
* and block until we can acquire the sem.
|
|
*/
|
|
waiter.task = current;
|
|
waiter.type = RWSEM_WAITING_FOR_WRITE;
|
|
waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
|
|
|
|
raw_spin_lock_irq(&sem->wait_lock);
|
|
|
|
/* account for this before adding a new element to the list */
|
|
wstate = list_empty(&sem->wait_list) ? WRITER_FIRST : WRITER_NOT_FIRST;
|
|
|
|
list_add_tail(&waiter.list, &sem->wait_list);
|
|
|
|
/* we're now waiting on the lock */
|
|
if (wstate == WRITER_NOT_FIRST) {
|
|
count = atomic_long_read(&sem->count);
|
|
|
|
/*
|
|
* If there were already threads queued before us and:
|
|
* 1) there are no no active locks, wake the front
|
|
* queued process(es) as the handoff bit might be set.
|
|
* 2) there are no active writers and some readers, the lock
|
|
* must be read owned; so we try to wake any read lock
|
|
* waiters that were queued ahead of us.
|
|
*/
|
|
if (count & RWSEM_WRITER_MASK)
|
|
goto wait;
|
|
|
|
rwsem_mark_wake(sem, (count & RWSEM_READER_MASK)
|
|
? RWSEM_WAKE_READERS
|
|
: RWSEM_WAKE_ANY, &wake_q);
|
|
|
|
if (!wake_q_empty(&wake_q)) {
|
|
/*
|
|
* We want to minimize wait_lock hold time especially
|
|
* when a large number of readers are to be woken up.
|
|
*/
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
wake_up_q(&wake_q);
|
|
wake_q_init(&wake_q); /* Used again, reinit */
|
|
raw_spin_lock_irq(&sem->wait_lock);
|
|
}
|
|
} else {
|
|
atomic_long_or(RWSEM_FLAG_WAITERS, &sem->count);
|
|
}
|
|
|
|
wait:
|
|
/* wait until we successfully acquire the lock */
|
|
set_current_state(state);
|
|
while (true) {
|
|
if (rwsem_try_write_lock(sem, wstate))
|
|
break;
|
|
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
|
|
/* Block until there are no active lockers. */
|
|
for (;;) {
|
|
if (signal_pending_state(state, current))
|
|
goto out_nolock;
|
|
|
|
schedule();
|
|
lockevent_inc(rwsem_sleep_writer);
|
|
set_current_state(state);
|
|
/*
|
|
* If HANDOFF bit is set, unconditionally do
|
|
* a trylock.
|
|
*/
|
|
if (wstate == WRITER_HANDOFF)
|
|
break;
|
|
|
|
if ((wstate == WRITER_NOT_FIRST) &&
|
|
(rwsem_first_waiter(sem) == &waiter))
|
|
wstate = WRITER_FIRST;
|
|
|
|
count = atomic_long_read(&sem->count);
|
|
if (!(count & RWSEM_LOCK_MASK))
|
|
break;
|
|
|
|
/*
|
|
* The setting of the handoff bit is deferred
|
|
* until rwsem_try_write_lock() is called.
|
|
*/
|
|
if ((wstate == WRITER_FIRST) && (rt_task(current) ||
|
|
time_after(jiffies, waiter.timeout))) {
|
|
wstate = WRITER_HANDOFF;
|
|
lockevent_inc(rwsem_wlock_handoff);
|
|
break;
|
|
}
|
|
}
|
|
|
|
raw_spin_lock_irq(&sem->wait_lock);
|
|
}
|
|
__set_current_state(TASK_RUNNING);
|
|
list_del(&waiter.list);
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
lockevent_inc(rwsem_wlock);
|
|
|
|
return ret;
|
|
|
|
out_nolock:
|
|
__set_current_state(TASK_RUNNING);
|
|
raw_spin_lock_irq(&sem->wait_lock);
|
|
list_del(&waiter.list);
|
|
|
|
if (unlikely(wstate == WRITER_HANDOFF))
|
|
atomic_long_add(-RWSEM_FLAG_HANDOFF, &sem->count);
|
|
|
|
if (list_empty(&sem->wait_list))
|
|
atomic_long_andnot(RWSEM_FLAG_WAITERS, &sem->count);
|
|
else
|
|
rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
wake_up_q(&wake_q);
|
|
lockevent_inc(rwsem_wlock_fail);
|
|
|
|
return ERR_PTR(-EINTR);
|
|
}
|
|
|
|
/*
|
|
* handle waking up a waiter on the semaphore
|
|
* - up_read/up_write has decremented the active part of count if we come here
|
|
*/
|
|
static struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem, long count)
|
|
{
|
|
unsigned long flags;
|
|
DEFINE_WAKE_Q(wake_q);
|
|
|
|
raw_spin_lock_irqsave(&sem->wait_lock, flags);
|
|
|
|
if (!list_empty(&sem->wait_list))
|
|
rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
|
|
|
|
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
|
|
wake_up_q(&wake_q);
|
|
|
|
return sem;
|
|
}
|
|
|
|
/*
|
|
* downgrade a write lock into a read lock
|
|
* - caller incremented waiting part of count and discovered it still negative
|
|
* - just wake up any readers at the front of the queue
|
|
*/
|
|
static struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
|
|
{
|
|
unsigned long flags;
|
|
DEFINE_WAKE_Q(wake_q);
|
|
|
|
raw_spin_lock_irqsave(&sem->wait_lock, flags);
|
|
|
|
if (!list_empty(&sem->wait_list))
|
|
rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);
|
|
|
|
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
|
|
wake_up_q(&wake_q);
|
|
|
|
return sem;
|
|
}
|
|
|
|
/*
|
|
* lock for reading
|
|
*/
|
|
inline void __down_read(struct rw_semaphore *sem)
|
|
{
|
|
if (unlikely(atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
|
|
&sem->count) & RWSEM_READ_FAILED_MASK)) {
|
|
rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE);
|
|
DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner &
|
|
RWSEM_READER_OWNED), sem);
|
|
} else {
|
|
rwsem_set_reader_owned(sem);
|
|
}
|
|
}
|
|
|
|
static inline int __down_read_killable(struct rw_semaphore *sem)
|
|
{
|
|
if (unlikely(atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
|
|
&sem->count) & RWSEM_READ_FAILED_MASK)) {
|
|
if (IS_ERR(rwsem_down_read_slowpath(sem, TASK_KILLABLE)))
|
|
return -EINTR;
|
|
DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner &
|
|
RWSEM_READER_OWNED), sem);
|
|
} else {
|
|
rwsem_set_reader_owned(sem);
|
|
}
|
|
return 0;
|
|
}
|
|
|
|
static inline int __down_read_trylock(struct rw_semaphore *sem)
|
|
{
|
|
/*
|
|
* Optimize for the case when the rwsem is not locked at all.
|
|
*/
|
|
long tmp = RWSEM_UNLOCKED_VALUE;
|
|
|
|
do {
|
|
if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
|
|
tmp + RWSEM_READER_BIAS)) {
|
|
rwsem_set_reader_owned(sem);
|
|
return 1;
|
|
}
|
|
} while (!(tmp & RWSEM_READ_FAILED_MASK));
|
|
return 0;
|
|
}
|
|
|
|
/*
|
|
* lock for writing
|
|
*/
|
|
static inline void __down_write(struct rw_semaphore *sem)
|
|
{
|
|
long tmp = RWSEM_UNLOCKED_VALUE;
|
|
|
|
if (unlikely(!atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
|
|
RWSEM_WRITER_LOCKED)))
|
|
rwsem_down_write_slowpath(sem, TASK_UNINTERRUPTIBLE);
|
|
rwsem_set_owner(sem);
|
|
}
|
|
|
|
static inline int __down_write_killable(struct rw_semaphore *sem)
|
|
{
|
|
long tmp = RWSEM_UNLOCKED_VALUE;
|
|
|
|
if (unlikely(!atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
|
|
RWSEM_WRITER_LOCKED))) {
|
|
if (IS_ERR(rwsem_down_write_slowpath(sem, TASK_KILLABLE)))
|
|
return -EINTR;
|
|
}
|
|
rwsem_set_owner(sem);
|
|
return 0;
|
|
}
|
|
|
|
static inline int __down_write_trylock(struct rw_semaphore *sem)
|
|
{
|
|
long tmp = RWSEM_UNLOCKED_VALUE;
|
|
|
|
if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
|
|
RWSEM_WRITER_LOCKED)) {
|
|
rwsem_set_owner(sem);
|
|
return true;
|
|
}
|
|
return false;
|
|
}
|
|
|
|
/*
|
|
* unlock after reading
|
|
*/
|
|
inline void __up_read(struct rw_semaphore *sem)
|
|
{
|
|
long tmp;
|
|
|
|
DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner & RWSEM_READER_OWNED), sem);
|
|
rwsem_clear_reader_owned(sem);
|
|
tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count);
|
|
if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS)) ==
|
|
RWSEM_FLAG_WAITERS))
|
|
rwsem_wake(sem, tmp);
|
|
}
|
|
|
|
/*
|
|
* unlock after writing
|
|
*/
|
|
static inline void __up_write(struct rw_semaphore *sem)
|
|
{
|
|
long tmp;
|
|
|
|
DEBUG_RWSEMS_WARN_ON(sem->owner != current, sem);
|
|
rwsem_clear_owner(sem);
|
|
tmp = atomic_long_fetch_add_release(-RWSEM_WRITER_LOCKED, &sem->count);
|
|
if (unlikely(tmp & RWSEM_FLAG_WAITERS))
|
|
rwsem_wake(sem, tmp);
|
|
}
|
|
|
|
/*
|
|
* downgrade write lock to read lock
|
|
*/
|
|
static inline void __downgrade_write(struct rw_semaphore *sem)
|
|
{
|
|
long tmp;
|
|
|
|
/*
|
|
* When downgrading from exclusive to shared ownership,
|
|
* anything inside the write-locked region cannot leak
|
|
* into the read side. In contrast, anything in the
|
|
* read-locked region is ok to be re-ordered into the
|
|
* write side. As such, rely on RELEASE semantics.
|
|
*/
|
|
DEBUG_RWSEMS_WARN_ON(sem->owner != current, sem);
|
|
tmp = atomic_long_fetch_add_release(
|
|
-RWSEM_WRITER_LOCKED+RWSEM_READER_BIAS, &sem->count);
|
|
rwsem_set_reader_owned(sem);
|
|
if (tmp & RWSEM_FLAG_WAITERS)
|
|
rwsem_downgrade_wake(sem);
|
|
}
|
|
|
|
/*
|
|
* lock for reading
|
|
*/
|
|
void __sched down_read(struct rw_semaphore *sem)
|
|
{
|
|
might_sleep();
|
|
rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
|
|
|
|
LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
|
|
}
|
|
EXPORT_SYMBOL(down_read);
|
|
|
|
int __sched down_read_killable(struct rw_semaphore *sem)
|
|
{
|
|
might_sleep();
|
|
rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
|
|
|
|
if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_killable)) {
|
|
rwsem_release(&sem->dep_map, 1, _RET_IP_);
|
|
return -EINTR;
|
|
}
|
|
|
|
return 0;
|
|
}
|
|
EXPORT_SYMBOL(down_read_killable);
|
|
|
|
/*
|
|
* trylock for reading -- returns 1 if successful, 0 if contention
|
|
*/
|
|
int down_read_trylock(struct rw_semaphore *sem)
|
|
{
|
|
int ret = __down_read_trylock(sem);
|
|
|
|
if (ret == 1)
|
|
rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
|
|
return ret;
|
|
}
|
|
EXPORT_SYMBOL(down_read_trylock);
|
|
|
|
/*
|
|
* lock for writing
|
|
*/
|
|
void __sched down_write(struct rw_semaphore *sem)
|
|
{
|
|
might_sleep();
|
|
rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
|
|
LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
|
|
}
|
|
EXPORT_SYMBOL(down_write);
|
|
|
|
/*
|
|
* lock for writing
|
|
*/
|
|
int __sched down_write_killable(struct rw_semaphore *sem)
|
|
{
|
|
might_sleep();
|
|
rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
|
|
|
|
if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
|
|
__down_write_killable)) {
|
|
rwsem_release(&sem->dep_map, 1, _RET_IP_);
|
|
return -EINTR;
|
|
}
|
|
|
|
return 0;
|
|
}
|
|
EXPORT_SYMBOL(down_write_killable);
|
|
|
|
/*
|
|
* trylock for writing -- returns 1 if successful, 0 if contention
|
|
*/
|
|
int down_write_trylock(struct rw_semaphore *sem)
|
|
{
|
|
int ret = __down_write_trylock(sem);
|
|
|
|
if (ret == 1)
|
|
rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_);
|
|
|
|
return ret;
|
|
}
|
|
EXPORT_SYMBOL(down_write_trylock);
|
|
|
|
/*
|
|
* release a read lock
|
|
*/
|
|
void up_read(struct rw_semaphore *sem)
|
|
{
|
|
rwsem_release(&sem->dep_map, 1, _RET_IP_);
|
|
__up_read(sem);
|
|
}
|
|
EXPORT_SYMBOL(up_read);
|
|
|
|
/*
|
|
* release a write lock
|
|
*/
|
|
void up_write(struct rw_semaphore *sem)
|
|
{
|
|
rwsem_release(&sem->dep_map, 1, _RET_IP_);
|
|
__up_write(sem);
|
|
}
|
|
EXPORT_SYMBOL(up_write);
|
|
|
|
/*
|
|
* downgrade write lock to read lock
|
|
*/
|
|
void downgrade_write(struct rw_semaphore *sem)
|
|
{
|
|
lock_downgrade(&sem->dep_map, _RET_IP_);
|
|
__downgrade_write(sem);
|
|
}
|
|
EXPORT_SYMBOL(downgrade_write);
|
|
|
|
#ifdef CONFIG_DEBUG_LOCK_ALLOC
|
|
|
|
void down_read_nested(struct rw_semaphore *sem, int subclass)
|
|
{
|
|
might_sleep();
|
|
rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
|
|
LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
|
|
}
|
|
EXPORT_SYMBOL(down_read_nested);
|
|
|
|
void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest)
|
|
{
|
|
might_sleep();
|
|
rwsem_acquire_nest(&sem->dep_map, 0, 0, nest, _RET_IP_);
|
|
LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
|
|
}
|
|
EXPORT_SYMBOL(_down_write_nest_lock);
|
|
|
|
void down_read_non_owner(struct rw_semaphore *sem)
|
|
{
|
|
might_sleep();
|
|
__down_read(sem);
|
|
__rwsem_set_reader_owned(sem, NULL);
|
|
}
|
|
EXPORT_SYMBOL(down_read_non_owner);
|
|
|
|
void down_write_nested(struct rw_semaphore *sem, int subclass)
|
|
{
|
|
might_sleep();
|
|
rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
|
|
LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
|
|
}
|
|
EXPORT_SYMBOL(down_write_nested);
|
|
|
|
int __sched down_write_killable_nested(struct rw_semaphore *sem, int subclass)
|
|
{
|
|
might_sleep();
|
|
rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
|
|
|
|
if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
|
|
__down_write_killable)) {
|
|
rwsem_release(&sem->dep_map, 1, _RET_IP_);
|
|
return -EINTR;
|
|
}
|
|
|
|
return 0;
|
|
}
|
|
EXPORT_SYMBOL(down_write_killable_nested);
|
|
|
|
void up_read_non_owner(struct rw_semaphore *sem)
|
|
{
|
|
DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner & RWSEM_READER_OWNED),
|
|
sem);
|
|
__up_read(sem);
|
|
}
|
|
EXPORT_SYMBOL(up_read_non_owner);
|
|
|
|
#endif
|