413 lines
14 KiB
C
413 lines
14 KiB
C
/*
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* Sleepable Read-Copy Update mechanism for mutual exclusion.
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
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*
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* Copyright (C) IBM Corporation, 2006
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*
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* Author: Paul McKenney <paulmck@us.ibm.com>
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*
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* For detailed explanation of Read-Copy Update mechanism see -
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* Documentation/RCU/ *.txt
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*
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*/
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#include <linux/export.h>
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#include <linux/mutex.h>
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#include <linux/percpu.h>
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#include <linux/preempt.h>
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#include <linux/rcupdate.h>
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#include <linux/sched.h>
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#include <linux/smp.h>
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#include <linux/delay.h>
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#include <linux/srcu.h>
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static int init_srcu_struct_fields(struct srcu_struct *sp)
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{
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sp->completed = 0;
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mutex_init(&sp->mutex);
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sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array);
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return sp->per_cpu_ref ? 0 : -ENOMEM;
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}
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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int __init_srcu_struct(struct srcu_struct *sp, const char *name,
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struct lock_class_key *key)
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{
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/* Don't re-initialize a lock while it is held. */
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debug_check_no_locks_freed((void *)sp, sizeof(*sp));
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lockdep_init_map(&sp->dep_map, name, key, 0);
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return init_srcu_struct_fields(sp);
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}
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EXPORT_SYMBOL_GPL(__init_srcu_struct);
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#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
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/**
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* init_srcu_struct - initialize a sleep-RCU structure
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* @sp: structure to initialize.
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*
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* Must invoke this on a given srcu_struct before passing that srcu_struct
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* to any other function. Each srcu_struct represents a separate domain
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* of SRCU protection.
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*/
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int init_srcu_struct(struct srcu_struct *sp)
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{
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return init_srcu_struct_fields(sp);
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}
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EXPORT_SYMBOL_GPL(init_srcu_struct);
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#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
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/*
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* Returns approximate total of the readers' ->seq[] values for the
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* rank of per-CPU counters specified by idx.
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*/
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static unsigned long srcu_readers_seq_idx(struct srcu_struct *sp, int idx)
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{
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int cpu;
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unsigned long sum = 0;
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unsigned long t;
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for_each_possible_cpu(cpu) {
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t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->seq[idx]);
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sum += t;
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}
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return sum;
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}
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/*
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* Returns approximate number of readers active on the specified rank
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* of the per-CPU ->c[] counters.
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*/
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static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx)
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{
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int cpu;
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unsigned long sum = 0;
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unsigned long t;
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for_each_possible_cpu(cpu) {
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t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]);
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sum += t;
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}
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return sum;
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}
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/*
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* Return true if the number of pre-existing readers is determined to
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* be stably zero. An example unstable zero can occur if the call
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* to srcu_readers_active_idx() misses an __srcu_read_lock() increment,
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* but due to task migration, sees the corresponding __srcu_read_unlock()
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* decrement. This can happen because srcu_readers_active_idx() takes
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* time to sum the array, and might in fact be interrupted or preempted
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* partway through the summation.
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*/
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static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
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{
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unsigned long seq;
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seq = srcu_readers_seq_idx(sp, idx);
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/*
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* The following smp_mb() A pairs with the smp_mb() B located in
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* __srcu_read_lock(). This pairing ensures that if an
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* __srcu_read_lock() increments its counter after the summation
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* in srcu_readers_active_idx(), then the corresponding SRCU read-side
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* critical section will see any changes made prior to the start
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* of the current SRCU grace period.
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*
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* Also, if the above call to srcu_readers_seq_idx() saw the
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* increment of ->seq[], then the call to srcu_readers_active_idx()
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* must see the increment of ->c[].
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*/
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smp_mb(); /* A */
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/*
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* Note that srcu_readers_active_idx() can incorrectly return
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* zero even though there is a pre-existing reader throughout.
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* To see this, suppose that task A is in a very long SRCU
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* read-side critical section that started on CPU 0, and that
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* no other reader exists, so that the sum of the counters
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* is equal to one. Then suppose that task B starts executing
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* srcu_readers_active_idx(), summing up to CPU 1, and then that
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* task C starts reading on CPU 0, so that its increment is not
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* summed, but finishes reading on CPU 2, so that its decrement
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* -is- summed. Then when task B completes its sum, it will
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* incorrectly get zero, despite the fact that task A has been
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* in its SRCU read-side critical section the whole time.
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*
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* We therefore do a validation step should srcu_readers_active_idx()
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* return zero.
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*/
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if (srcu_readers_active_idx(sp, idx) != 0)
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return false;
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/*
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* The remainder of this function is the validation step.
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* The following smp_mb() D pairs with the smp_mb() C in
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* __srcu_read_unlock(). If the __srcu_read_unlock() was seen
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* by srcu_readers_active_idx() above, then any destructive
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* operation performed after the grace period will happen after
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* the corresponding SRCU read-side critical section.
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*
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* Note that there can be at most NR_CPUS worth of readers using
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* the old index, which is not enough to overflow even a 32-bit
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* integer. (Yes, this does mean that systems having more than
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* a billion or so CPUs need to be 64-bit systems.) Therefore,
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* the sum of the ->seq[] counters cannot possibly overflow.
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* Therefore, the only way that the return values of the two
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* calls to srcu_readers_seq_idx() can be equal is if there were
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* no increments of the corresponding rank of ->seq[] counts
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* in the interim. But the missed-increment scenario laid out
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* above includes an increment of the ->seq[] counter by
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* the corresponding __srcu_read_lock(). Therefore, if this
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* scenario occurs, the return values from the two calls to
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* srcu_readers_seq_idx() will differ, and thus the validation
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* step below suffices.
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*/
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smp_mb(); /* D */
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return srcu_readers_seq_idx(sp, idx) == seq;
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}
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/**
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* srcu_readers_active - returns approximate number of readers.
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* @sp: which srcu_struct to count active readers (holding srcu_read_lock).
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*
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* Note that this is not an atomic primitive, and can therefore suffer
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* severe errors when invoked on an active srcu_struct. That said, it
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* can be useful as an error check at cleanup time.
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*/
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static int srcu_readers_active(struct srcu_struct *sp)
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{
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int cpu;
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unsigned long sum = 0;
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for_each_possible_cpu(cpu) {
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sum += ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[0]);
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sum += ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[1]);
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}
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return sum;
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}
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/**
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* cleanup_srcu_struct - deconstruct a sleep-RCU structure
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* @sp: structure to clean up.
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*
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* Must invoke this after you are finished using a given srcu_struct that
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* was initialized via init_srcu_struct(), else you leak memory.
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*/
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void cleanup_srcu_struct(struct srcu_struct *sp)
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{
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int sum;
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sum = srcu_readers_active(sp);
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WARN_ON(sum); /* Leakage unless caller handles error. */
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if (sum != 0)
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return;
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free_percpu(sp->per_cpu_ref);
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sp->per_cpu_ref = NULL;
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}
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EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
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/*
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* Counts the new reader in the appropriate per-CPU element of the
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* srcu_struct. Must be called from process context.
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* Returns an index that must be passed to the matching srcu_read_unlock().
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*/
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int __srcu_read_lock(struct srcu_struct *sp)
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{
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int idx;
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preempt_disable();
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idx = rcu_dereference_index_check(sp->completed,
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rcu_read_lock_sched_held()) & 0x1;
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ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) += 1;
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smp_mb(); /* B */ /* Avoid leaking the critical section. */
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ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->seq[idx]) += 1;
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preempt_enable();
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return idx;
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}
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EXPORT_SYMBOL_GPL(__srcu_read_lock);
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/*
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* Removes the count for the old reader from the appropriate per-CPU
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* element of the srcu_struct. Note that this may well be a different
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* CPU than that which was incremented by the corresponding srcu_read_lock().
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* Must be called from process context.
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*/
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void __srcu_read_unlock(struct srcu_struct *sp, int idx)
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{
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preempt_disable();
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smp_mb(); /* C */ /* Avoid leaking the critical section. */
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ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) -= 1;
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preempt_enable();
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}
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EXPORT_SYMBOL_GPL(__srcu_read_unlock);
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/*
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* We use an adaptive strategy for synchronize_srcu() and especially for
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* synchronize_srcu_expedited(). We spin for a fixed time period
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* (defined below) to allow SRCU readers to exit their read-side critical
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* sections. If there are still some readers after 10 microseconds,
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* we repeatedly block for 1-millisecond time periods. This approach
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* has done well in testing, so there is no need for a config parameter.
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*/
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#define SYNCHRONIZE_SRCU_READER_DELAY 5
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#define SYNCHRONIZE_SRCU_TRYCOUNT 2
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#define SYNCHRONIZE_SRCU_EXP_TRYCOUNT 12
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/*
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* Wait until all pre-existing readers complete. Such readers
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* will have used the index specified by "idx".
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*/
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static void wait_idx(struct srcu_struct *sp, int idx, int trycount)
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{
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/*
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* SRCU read-side critical sections are normally short, so wait
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* a small amount of time before possibly blocking.
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*/
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if (!srcu_readers_active_idx_check(sp, idx)) {
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udelay(SYNCHRONIZE_SRCU_READER_DELAY);
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while (!srcu_readers_active_idx_check(sp, idx)) {
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if (trycount > 0) {
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trycount--;
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udelay(SYNCHRONIZE_SRCU_READER_DELAY);
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} else
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schedule_timeout_interruptible(1);
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}
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}
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}
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static void srcu_flip(struct srcu_struct *sp)
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{
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sp->completed++;
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}
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/*
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* Helper function for synchronize_srcu() and synchronize_srcu_expedited().
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*/
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static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
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{
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int busy_idx;
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rcu_lockdep_assert(!lock_is_held(&sp->dep_map) &&
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!lock_is_held(&rcu_bh_lock_map) &&
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!lock_is_held(&rcu_lock_map) &&
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!lock_is_held(&rcu_sched_lock_map),
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"Illegal synchronize_srcu() in same-type SRCU (or RCU) read-side critical section");
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mutex_lock(&sp->mutex);
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busy_idx = sp->completed & 0X1UL;
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/*
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* If we recently flipped the index, there will be some readers
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* using idx=0 and others using idx=1. Therefore, two calls to
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* wait_idx()s suffice to ensure that all pre-existing readers
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* have completed:
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*
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* __synchronize_srcu() {
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* wait_idx(sp, 0, trycount);
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* wait_idx(sp, 1, trycount);
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* }
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*
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* Starvation is prevented by the fact that we flip the index.
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* While we wait on one index to clear out, almost all new readers
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* will be using the other index. The number of new readers using the
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* index we are waiting on is sharply bounded by roughly the number
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* of CPUs.
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*
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* How can new readers possibly using the old pre-flip value of
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* the index? Consider the following sequence of events:
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*
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* Suppose that during the previous grace period, a reader
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* picked up the old value of the index, but did not increment
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* its counter until after the previous instance of
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* __synchronize_srcu() did the counter summation and recheck.
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* That previous grace period was OK because the reader did
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* not start until after the grace period started, so the grace
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* period was not obligated to wait for that reader.
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*
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* However, this sequence of events is quite improbable, so
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* this call to wait_idx(), which waits on really old readers
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* describe in this comment above, will almost never need to wait.
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*/
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wait_idx(sp, 1 - busy_idx, trycount);
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/* Flip the index to avoid reader-induced starvation. */
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srcu_flip(sp);
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/* Wait for recent pre-existing readers. */
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wait_idx(sp, busy_idx, trycount);
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mutex_unlock(&sp->mutex);
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}
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/**
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* synchronize_srcu - wait for prior SRCU read-side critical-section completion
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* @sp: srcu_struct with which to synchronize.
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*
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* Flip the completed counter, and wait for the old count to drain to zero.
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* As with classic RCU, the updater must use some separate means of
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* synchronizing concurrent updates. Can block; must be called from
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* process context.
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*
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* Note that it is illegal to call synchronize_srcu() from the corresponding
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* SRCU read-side critical section; doing so will result in deadlock.
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* However, it is perfectly legal to call synchronize_srcu() on one
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* srcu_struct from some other srcu_struct's read-side critical section.
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*/
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void synchronize_srcu(struct srcu_struct *sp)
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{
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__synchronize_srcu(sp, SYNCHRONIZE_SRCU_TRYCOUNT);
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}
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EXPORT_SYMBOL_GPL(synchronize_srcu);
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/**
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* synchronize_srcu_expedited - Brute-force SRCU grace period
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* @sp: srcu_struct with which to synchronize.
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*
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* Wait for an SRCU grace period to elapse, but be more aggressive about
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* spinning rather than blocking when waiting.
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*
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* Note that it is illegal to call this function while holding any lock
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* that is acquired by a CPU-hotplug notifier. It is also illegal to call
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* synchronize_srcu_expedited() from the corresponding SRCU read-side
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* critical section; doing so will result in deadlock. However, it is
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* perfectly legal to call synchronize_srcu_expedited() on one srcu_struct
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* from some other srcu_struct's read-side critical section, as long as
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* the resulting graph of srcu_structs is acyclic.
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*/
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void synchronize_srcu_expedited(struct srcu_struct *sp)
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{
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__synchronize_srcu(sp, SYNCHRONIZE_SRCU_EXP_TRYCOUNT);
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}
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EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
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/**
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* srcu_batches_completed - return batches completed.
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* @sp: srcu_struct on which to report batch completion.
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*
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* Report the number of batches, correlated with, but not necessarily
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* precisely the same as, the number of grace periods that have elapsed.
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*/
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long srcu_batches_completed(struct srcu_struct *sp)
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{
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return sp->completed;
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}
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EXPORT_SYMBOL_GPL(srcu_batches_completed);
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