238 lines
7 KiB
C
238 lines
7 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
|
|
/* rwsem.h: R/W semaphores implemented using XADD/CMPXCHG for i486+
|
|
*
|
|
* Written by David Howells (dhowells@redhat.com).
|
|
*
|
|
* Derived from asm-x86/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 consecutive readers at the
|
|
* front, then they'll all be woken up, but no other readers will be.
|
|
*/
|
|
|
|
#ifndef _ASM_X86_RWSEM_H
|
|
#define _ASM_X86_RWSEM_H
|
|
|
|
#ifndef _LINUX_RWSEM_H
|
|
#error "please don't include asm/rwsem.h directly, use linux/rwsem.h instead"
|
|
#endif
|
|
|
|
#ifdef __KERNEL__
|
|
#include <asm/asm.h>
|
|
|
|
/*
|
|
* The bias values and the counter type limits the number of
|
|
* potential readers/writers to 32767 for 32 bits and 2147483647
|
|
* for 64 bits.
|
|
*/
|
|
|
|
#ifdef CONFIG_X86_64
|
|
# define RWSEM_ACTIVE_MASK 0xffffffffL
|
|
#else
|
|
# define RWSEM_ACTIVE_MASK 0x0000ffffL
|
|
#endif
|
|
|
|
#define RWSEM_UNLOCKED_VALUE 0x00000000L
|
|
#define RWSEM_ACTIVE_BIAS 0x00000001L
|
|
#define RWSEM_WAITING_BIAS (-RWSEM_ACTIVE_MASK-1)
|
|
#define RWSEM_ACTIVE_READ_BIAS RWSEM_ACTIVE_BIAS
|
|
#define RWSEM_ACTIVE_WRITE_BIAS (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
|
|
|
|
/*
|
|
* lock for reading
|
|
*/
|
|
#define ____down_read(sem, slow_path) \
|
|
({ \
|
|
struct rw_semaphore* ret; \
|
|
asm volatile("# beginning down_read\n\t" \
|
|
LOCK_PREFIX _ASM_INC "(%[sem])\n\t" \
|
|
/* adds 0x00000001 */ \
|
|
" jns 1f\n" \
|
|
" call " slow_path "\n" \
|
|
"1:\n\t" \
|
|
"# ending down_read\n\t" \
|
|
: "+m" (sem->count), "=a" (ret), \
|
|
ASM_CALL_CONSTRAINT \
|
|
: [sem] "a" (sem) \
|
|
: "memory", "cc"); \
|
|
ret; \
|
|
})
|
|
|
|
static inline void __down_read(struct rw_semaphore *sem)
|
|
{
|
|
____down_read(sem, "call_rwsem_down_read_failed");
|
|
}
|
|
|
|
static inline int __down_read_killable(struct rw_semaphore *sem)
|
|
{
|
|
if (IS_ERR(____down_read(sem, "call_rwsem_down_read_failed_killable")))
|
|
return -EINTR;
|
|
return 0;
|
|
}
|
|
|
|
/*
|
|
* trylock for reading -- returns 1 if successful, 0 if contention
|
|
*/
|
|
static inline bool __down_read_trylock(struct rw_semaphore *sem)
|
|
{
|
|
long result, tmp;
|
|
asm volatile("# beginning __down_read_trylock\n\t"
|
|
" mov %[count],%[result]\n\t"
|
|
"1:\n\t"
|
|
" mov %[result],%[tmp]\n\t"
|
|
" add %[inc],%[tmp]\n\t"
|
|
" jle 2f\n\t"
|
|
LOCK_PREFIX " cmpxchg %[tmp],%[count]\n\t"
|
|
" jnz 1b\n\t"
|
|
"2:\n\t"
|
|
"# ending __down_read_trylock\n\t"
|
|
: [count] "+m" (sem->count), [result] "=&a" (result),
|
|
[tmp] "=&r" (tmp)
|
|
: [inc] "i" (RWSEM_ACTIVE_READ_BIAS)
|
|
: "memory", "cc");
|
|
return result >= 0;
|
|
}
|
|
|
|
/*
|
|
* lock for writing
|
|
*/
|
|
#define ____down_write(sem, slow_path) \
|
|
({ \
|
|
long tmp; \
|
|
struct rw_semaphore* ret; \
|
|
\
|
|
asm volatile("# beginning down_write\n\t" \
|
|
LOCK_PREFIX " xadd %[tmp],(%[sem])\n\t" \
|
|
/* adds 0xffff0001, returns the old value */ \
|
|
" test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t" \
|
|
/* was the active mask 0 before? */\
|
|
" jz 1f\n" \
|
|
" call " slow_path "\n" \
|
|
"1:\n" \
|
|
"# ending down_write" \
|
|
: "+m" (sem->count), [tmp] "=d" (tmp), \
|
|
"=a" (ret), ASM_CALL_CONSTRAINT \
|
|
: [sem] "a" (sem), "[tmp]" (RWSEM_ACTIVE_WRITE_BIAS) \
|
|
: "memory", "cc"); \
|
|
ret; \
|
|
})
|
|
|
|
static inline void __down_write(struct rw_semaphore *sem)
|
|
{
|
|
____down_write(sem, "call_rwsem_down_write_failed");
|
|
}
|
|
|
|
static inline int __down_write_killable(struct rw_semaphore *sem)
|
|
{
|
|
if (IS_ERR(____down_write(sem, "call_rwsem_down_write_failed_killable")))
|
|
return -EINTR;
|
|
|
|
return 0;
|
|
}
|
|
|
|
/*
|
|
* trylock for writing -- returns 1 if successful, 0 if contention
|
|
*/
|
|
static inline bool __down_write_trylock(struct rw_semaphore *sem)
|
|
{
|
|
bool result;
|
|
long tmp0, tmp1;
|
|
asm volatile("# beginning __down_write_trylock\n\t"
|
|
" mov %[count],%[tmp0]\n\t"
|
|
"1:\n\t"
|
|
" test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t"
|
|
/* was the active mask 0 before? */
|
|
" jnz 2f\n\t"
|
|
" mov %[tmp0],%[tmp1]\n\t"
|
|
" add %[inc],%[tmp1]\n\t"
|
|
LOCK_PREFIX " cmpxchg %[tmp1],%[count]\n\t"
|
|
" jnz 1b\n\t"
|
|
"2:\n\t"
|
|
CC_SET(e)
|
|
"# ending __down_write_trylock\n\t"
|
|
: [count] "+m" (sem->count), [tmp0] "=&a" (tmp0),
|
|
[tmp1] "=&r" (tmp1), CC_OUT(e) (result)
|
|
: [inc] "er" (RWSEM_ACTIVE_WRITE_BIAS)
|
|
: "memory");
|
|
return result;
|
|
}
|
|
|
|
/*
|
|
* unlock after reading
|
|
*/
|
|
static inline void __up_read(struct rw_semaphore *sem)
|
|
{
|
|
long tmp;
|
|
asm volatile("# beginning __up_read\n\t"
|
|
LOCK_PREFIX " xadd %[tmp],(%[sem])\n\t"
|
|
/* subtracts 1, returns the old value */
|
|
" jns 1f\n\t"
|
|
" call call_rwsem_wake\n" /* expects old value in %edx */
|
|
"1:\n"
|
|
"# ending __up_read\n"
|
|
: "+m" (sem->count), [tmp] "=d" (tmp)
|
|
: [sem] "a" (sem), "[tmp]" (-RWSEM_ACTIVE_READ_BIAS)
|
|
: "memory", "cc");
|
|
}
|
|
|
|
/*
|
|
* unlock after writing
|
|
*/
|
|
static inline void __up_write(struct rw_semaphore *sem)
|
|
{
|
|
long tmp;
|
|
asm volatile("# beginning __up_write\n\t"
|
|
LOCK_PREFIX " xadd %[tmp],(%[sem])\n\t"
|
|
/* subtracts 0xffff0001, returns the old value */
|
|
" jns 1f\n\t"
|
|
" call call_rwsem_wake\n" /* expects old value in %edx */
|
|
"1:\n\t"
|
|
"# ending __up_write\n"
|
|
: "+m" (sem->count), [tmp] "=d" (tmp)
|
|
: [sem] "a" (sem), "[tmp]" (-RWSEM_ACTIVE_WRITE_BIAS)
|
|
: "memory", "cc");
|
|
}
|
|
|
|
/*
|
|
* downgrade write lock to read lock
|
|
*/
|
|
static inline void __downgrade_write(struct rw_semaphore *sem)
|
|
{
|
|
asm volatile("# beginning __downgrade_write\n\t"
|
|
LOCK_PREFIX _ASM_ADD "%[inc],(%[sem])\n\t"
|
|
/*
|
|
* transitions 0xZZZZ0001 -> 0xYYYY0001 (i386)
|
|
* 0xZZZZZZZZ00000001 -> 0xYYYYYYYY00000001 (x86_64)
|
|
*/
|
|
" jns 1f\n\t"
|
|
" call call_rwsem_downgrade_wake\n"
|
|
"1:\n\t"
|
|
"# ending __downgrade_write\n"
|
|
: "+m" (sem->count)
|
|
: [sem] "a" (sem), [inc] "er" (-RWSEM_WAITING_BIAS)
|
|
: "memory", "cc");
|
|
}
|
|
|
|
#endif /* __KERNEL__ */
|
|
#endif /* _ASM_X86_RWSEM_H */
|