294 lines
7.8 KiB
C
294 lines
7.8 KiB
C
/* rwsem.h: R/W semaphores implemented using XADD/CMPXCHG for i486+
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*
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* Written by David Howells (dhowells@redhat.com).
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*
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* Derived from asm-i386/semaphore.h
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*
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*
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* The MSW of the count is the negated number of active writers and waiting
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* lockers, and the LSW is the total number of active locks
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*
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* The lock count is initialized to 0 (no active and no waiting lockers).
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*
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* When a writer subtracts WRITE_BIAS, it'll get 0xffff0001 for the case of an
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* uncontended lock. This can be determined because XADD returns the old value.
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* Readers increment by 1 and see a positive value when uncontended, negative
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* if there are writers (and maybe) readers waiting (in which case it goes to
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* sleep).
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*
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* The value of WAITING_BIAS supports up to 32766 waiting processes. This can
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* be extended to 65534 by manually checking the whole MSW rather than relying
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* on the S flag.
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*
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* The value of ACTIVE_BIAS supports up to 65535 active processes.
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*
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* This should be totally fair - if anything is waiting, a process that wants a
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* lock will go to the back of the queue. When the currently active lock is
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* released, if there's a writer at the front of the queue, then that and only
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* that will be woken up; if there's a bunch of consequtive readers at the
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* front, then they'll all be woken up, but no other readers will be.
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*/
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#ifndef _I386_RWSEM_H
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#define _I386_RWSEM_H
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#ifndef _LINUX_RWSEM_H
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#error "please don't include asm/rwsem.h directly, use linux/rwsem.h instead"
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#endif
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#ifdef __KERNEL__
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#include <linux/list.h>
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#include <linux/spinlock.h>
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struct rwsem_waiter;
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extern struct rw_semaphore *FASTCALL(rwsem_down_read_failed(struct rw_semaphore *sem));
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extern struct rw_semaphore *FASTCALL(rwsem_down_write_failed(struct rw_semaphore *sem));
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extern struct rw_semaphore *FASTCALL(rwsem_wake(struct rw_semaphore *));
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extern struct rw_semaphore *FASTCALL(rwsem_downgrade_wake(struct rw_semaphore *sem));
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/*
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* the semaphore definition
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*/
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struct rw_semaphore {
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signed long count;
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#define RWSEM_UNLOCKED_VALUE 0x00000000
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#define RWSEM_ACTIVE_BIAS 0x00000001
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#define RWSEM_ACTIVE_MASK 0x0000ffff
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#define RWSEM_WAITING_BIAS (-0x00010000)
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#define RWSEM_ACTIVE_READ_BIAS RWSEM_ACTIVE_BIAS
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#define RWSEM_ACTIVE_WRITE_BIAS (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
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spinlock_t wait_lock;
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struct list_head wait_list;
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#if RWSEM_DEBUG
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int debug;
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#endif
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};
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/*
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* initialisation
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*/
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#if RWSEM_DEBUG
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#define __RWSEM_DEBUG_INIT , 0
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#else
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#define __RWSEM_DEBUG_INIT /* */
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#endif
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#define __RWSEM_INITIALIZER(name) \
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{ RWSEM_UNLOCKED_VALUE, SPIN_LOCK_UNLOCKED, LIST_HEAD_INIT((name).wait_list) \
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__RWSEM_DEBUG_INIT }
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#define DECLARE_RWSEM(name) \
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struct rw_semaphore name = __RWSEM_INITIALIZER(name)
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static inline void init_rwsem(struct rw_semaphore *sem)
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{
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sem->count = RWSEM_UNLOCKED_VALUE;
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spin_lock_init(&sem->wait_lock);
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INIT_LIST_HEAD(&sem->wait_list);
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#if RWSEM_DEBUG
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sem->debug = 0;
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#endif
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}
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/*
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* lock for reading
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*/
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static inline void __down_read(struct rw_semaphore *sem)
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{
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__asm__ __volatile__(
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"# beginning down_read\n\t"
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LOCK_PREFIX " incl (%%eax)\n\t" /* adds 0x00000001, returns the old value */
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" js 2f\n\t" /* jump if we weren't granted the lock */
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"1:\n\t"
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LOCK_SECTION_START("")
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"2:\n\t"
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" pushl %%ecx\n\t"
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" pushl %%edx\n\t"
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" call rwsem_down_read_failed\n\t"
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" popl %%edx\n\t"
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" popl %%ecx\n\t"
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" jmp 1b\n"
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LOCK_SECTION_END
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"# ending down_read\n\t"
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: "=m"(sem->count)
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: "a"(sem), "m"(sem->count)
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: "memory", "cc");
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}
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/*
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* trylock for reading -- returns 1 if successful, 0 if contention
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*/
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static inline int __down_read_trylock(struct rw_semaphore *sem)
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{
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__s32 result, tmp;
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__asm__ __volatile__(
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"# beginning __down_read_trylock\n\t"
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" movl %0,%1\n\t"
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"1:\n\t"
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" movl %1,%2\n\t"
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" addl %3,%2\n\t"
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" jle 2f\n\t"
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LOCK_PREFIX " cmpxchgl %2,%0\n\t"
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" jnz 1b\n\t"
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"2:\n\t"
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"# ending __down_read_trylock\n\t"
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: "+m"(sem->count), "=&a"(result), "=&r"(tmp)
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: "i"(RWSEM_ACTIVE_READ_BIAS)
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: "memory", "cc");
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return result>=0 ? 1 : 0;
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}
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/*
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* lock for writing
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*/
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static inline void __down_write(struct rw_semaphore *sem)
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{
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int tmp;
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tmp = RWSEM_ACTIVE_WRITE_BIAS;
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__asm__ __volatile__(
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"# beginning down_write\n\t"
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LOCK_PREFIX " xadd %%edx,(%%eax)\n\t" /* subtract 0x0000ffff, returns the old value */
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" testl %%edx,%%edx\n\t" /* was the count 0 before? */
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" jnz 2f\n\t" /* jump if we weren't granted the lock */
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"1:\n\t"
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LOCK_SECTION_START("")
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"2:\n\t"
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" pushl %%ecx\n\t"
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" call rwsem_down_write_failed\n\t"
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" popl %%ecx\n\t"
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" jmp 1b\n"
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LOCK_SECTION_END
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"# ending down_write"
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: "=m"(sem->count), "=d"(tmp)
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: "a"(sem), "1"(tmp), "m"(sem->count)
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: "memory", "cc");
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}
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/*
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* trylock for writing -- returns 1 if successful, 0 if contention
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*/
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static inline int __down_write_trylock(struct rw_semaphore *sem)
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{
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signed long ret = cmpxchg(&sem->count,
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RWSEM_UNLOCKED_VALUE,
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RWSEM_ACTIVE_WRITE_BIAS);
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if (ret == RWSEM_UNLOCKED_VALUE)
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return 1;
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return 0;
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}
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/*
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* unlock after reading
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*/
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static inline void __up_read(struct rw_semaphore *sem)
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{
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__s32 tmp = -RWSEM_ACTIVE_READ_BIAS;
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__asm__ __volatile__(
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"# beginning __up_read\n\t"
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LOCK_PREFIX " xadd %%edx,(%%eax)\n\t" /* subtracts 1, returns the old value */
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" js 2f\n\t" /* jump if the lock is being waited upon */
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"1:\n\t"
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LOCK_SECTION_START("")
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"2:\n\t"
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" decw %%dx\n\t" /* do nothing if still outstanding active readers */
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" jnz 1b\n\t"
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" pushl %%ecx\n\t"
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" call rwsem_wake\n\t"
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" popl %%ecx\n\t"
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" jmp 1b\n"
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LOCK_SECTION_END
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"# ending __up_read\n"
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: "=m"(sem->count), "=d"(tmp)
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: "a"(sem), "1"(tmp), "m"(sem->count)
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: "memory", "cc");
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}
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/*
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* unlock after writing
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*/
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static inline void __up_write(struct rw_semaphore *sem)
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{
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__asm__ __volatile__(
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"# beginning __up_write\n\t"
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" movl %2,%%edx\n\t"
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LOCK_PREFIX " xaddl %%edx,(%%eax)\n\t" /* tries to transition 0xffff0001 -> 0x00000000 */
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" jnz 2f\n\t" /* jump if the lock is being waited upon */
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"1:\n\t"
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LOCK_SECTION_START("")
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"2:\n\t"
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" decw %%dx\n\t" /* did the active count reduce to 0? */
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" jnz 1b\n\t" /* jump back if not */
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" pushl %%ecx\n\t"
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" call rwsem_wake\n\t"
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" popl %%ecx\n\t"
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" jmp 1b\n"
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LOCK_SECTION_END
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"# ending __up_write\n"
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: "=m"(sem->count)
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: "a"(sem), "i"(-RWSEM_ACTIVE_WRITE_BIAS), "m"(sem->count)
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: "memory", "cc", "edx");
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}
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/*
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* downgrade write lock to read lock
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*/
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static inline void __downgrade_write(struct rw_semaphore *sem)
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{
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__asm__ __volatile__(
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"# beginning __downgrade_write\n\t"
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LOCK_PREFIX " addl %2,(%%eax)\n\t" /* transitions 0xZZZZ0001 -> 0xYYYY0001 */
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" js 2f\n\t" /* jump if the lock is being waited upon */
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"1:\n\t"
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LOCK_SECTION_START("")
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"2:\n\t"
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" pushl %%ecx\n\t"
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" pushl %%edx\n\t"
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" call rwsem_downgrade_wake\n\t"
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" popl %%edx\n\t"
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" popl %%ecx\n\t"
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" jmp 1b\n"
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LOCK_SECTION_END
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"# ending __downgrade_write\n"
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: "=m"(sem->count)
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: "a"(sem), "i"(-RWSEM_WAITING_BIAS), "m"(sem->count)
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: "memory", "cc");
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}
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/*
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* implement atomic add functionality
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*/
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static inline void rwsem_atomic_add(int delta, struct rw_semaphore *sem)
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{
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__asm__ __volatile__(
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LOCK_PREFIX "addl %1,%0"
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: "=m"(sem->count)
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: "ir"(delta), "m"(sem->count));
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}
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/*
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* implement exchange and add functionality
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*/
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static inline int rwsem_atomic_update(int delta, struct rw_semaphore *sem)
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{
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int tmp = delta;
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__asm__ __volatile__(
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LOCK_PREFIX "xadd %0,(%2)"
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: "+r"(tmp), "=m"(sem->count)
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: "r"(sem), "m"(sem->count)
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: "memory");
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return tmp+delta;
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}
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static inline int rwsem_is_locked(struct rw_semaphore *sem)
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{
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return (sem->count != 0);
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}
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#endif /* __KERNEL__ */
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#endif /* _I386_RWSEM_H */
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