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