forked from Minki/linux
620a6fd185
Add three (generic) mutex fastpath implementations. The mutex-xchg.h implementation is atomic_xchg() based, and should work fine on every architecture. The mutex-dec.h implementation is atomic_dec_return() based - this one too should work on every architecture, but might not perform the most optimally on architectures that have no atomic-dec/inc instructions. The mutex-null.h implementation forces all calls into the slowpath. This is used for mutex debugging, but it can also be used on platforms that do not want (or need) a fastpath at all. Signed-off-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Arjan van de Ven <arjan@infradead.org>
118 lines
3.7 KiB
C
118 lines
3.7 KiB
C
/*
|
|
* asm-generic/mutex-xchg.h
|
|
*
|
|
* Generic implementation of the mutex fastpath, based on xchg().
|
|
*
|
|
* NOTE: An xchg based implementation is less optimal than an atomic
|
|
* decrement/increment based implementation. If your architecture
|
|
* has a reasonable atomic dec/inc then you should probably use
|
|
* asm-generic/mutex-dec.h instead, or you could open-code an
|
|
* optimized version in asm/mutex.h.
|
|
*/
|
|
#ifndef _ASM_GENERIC_MUTEX_XCHG_H
|
|
#define _ASM_GENERIC_MUTEX_XCHG_H
|
|
|
|
/**
|
|
* __mutex_fastpath_lock - try to take the lock by moving the count
|
|
* from 1 to a 0 value
|
|
* @count: pointer of type atomic_t
|
|
* @fail_fn: function to call if the original value was not 1
|
|
*
|
|
* Change the count from 1 to a value lower than 1, and call <fail_fn> if it
|
|
* wasn't 1 originally. This function MUST leave the value lower than 1
|
|
* even when the "1" assertion wasn't true.
|
|
*/
|
|
#define __mutex_fastpath_lock(count, fail_fn) \
|
|
do { \
|
|
if (unlikely(atomic_xchg(count, 0) != 1)) \
|
|
fail_fn(count); \
|
|
else \
|
|
smp_mb(); \
|
|
} while (0)
|
|
|
|
|
|
/**
|
|
* __mutex_fastpath_lock_retval - try to take the lock by moving the count
|
|
* from 1 to a 0 value
|
|
* @count: pointer of type atomic_t
|
|
* @fail_fn: function to call if the original value was not 1
|
|
*
|
|
* Change the count from 1 to a value lower than 1, and call <fail_fn> if it
|
|
* wasn't 1 originally. This function returns 0 if the fastpath succeeds,
|
|
* or anything the slow path function returns
|
|
*/
|
|
static inline int
|
|
__mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *))
|
|
{
|
|
if (unlikely(atomic_xchg(count, 0) != 1))
|
|
return fail_fn(count);
|
|
else {
|
|
smp_mb();
|
|
return 0;
|
|
}
|
|
}
|
|
|
|
/**
|
|
* __mutex_fastpath_unlock - try to promote the mutex from 0 to 1
|
|
* @count: pointer of type atomic_t
|
|
* @fail_fn: function to call if the original value was not 0
|
|
*
|
|
* try to promote the mutex from 0 to 1. if it wasn't 0, call <function>
|
|
* In the failure case, this function is allowed to either set the value to
|
|
* 1, or to set it to a value lower than one.
|
|
* If the implementation sets it to a value of lower than one, the
|
|
* __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs
|
|
* to return 0 otherwise.
|
|
*/
|
|
#define __mutex_fastpath_unlock(count, fail_fn) \
|
|
do { \
|
|
smp_mb(); \
|
|
if (unlikely(atomic_xchg(count, 1) != 0)) \
|
|
fail_fn(count); \
|
|
} while (0)
|
|
|
|
#define __mutex_slowpath_needs_to_unlock() 0
|
|
|
|
/**
|
|
* __mutex_fastpath_trylock - try to acquire the mutex, without waiting
|
|
*
|
|
* @count: pointer of type atomic_t
|
|
* @fail_fn: spinlock based trylock implementation
|
|
*
|
|
* Change the count from 1 to a value lower than 1, and return 0 (failure)
|
|
* if it wasn't 1 originally, or return 1 (success) otherwise. This function
|
|
* MUST leave the value lower than 1 even when the "1" assertion wasn't true.
|
|
* Additionally, if the value was < 0 originally, this function must not leave
|
|
* it to 0 on failure.
|
|
*
|
|
* If the architecture has no effective trylock variant, it should call the
|
|
* <fail_fn> spinlock-based trylock variant unconditionally.
|
|
*/
|
|
static inline int
|
|
__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
|
|
{
|
|
int prev = atomic_xchg(count, 0);
|
|
|
|
if (unlikely(prev < 0)) {
|
|
/*
|
|
* The lock was marked contended so we must restore that
|
|
* state. If while doing so we get back a prev value of 1
|
|
* then we just own it.
|
|
*
|
|
* [ In the rare case of the mutex going to 1, to 0, to -1
|
|
* and then back to 0 in this few-instructions window,
|
|
* this has the potential to trigger the slowpath for the
|
|
* owner's unlock path needlessly, but that's not a problem
|
|
* in practice. ]
|
|
*/
|
|
prev = atomic_xchg(count, prev);
|
|
if (prev < 0)
|
|
prev = 0;
|
|
}
|
|
smp_mb();
|
|
|
|
return prev;
|
|
}
|
|
|
|
#endif
|