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/*
* include/asm-generic/mutex-xchg.h
*
* Generic implementation of the mutex fastpath, based on xchg().
*
* NOTE: An xchg based implementation might be 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.
*/
static inline void
__mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *))
{
if (unlikely(atomic_xchg(count, 0) != 1))
/*
* We failed to acquire the lock, so mark it contended
* to ensure that any waiting tasks are woken up by the
* unlock slow path.
*/
if (likely(atomic_xchg(count, -1) != 1))
fail_fn(count);
}
/**
* __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
*
* Change the count from 1 to a value lower than 1. This function returns 0
* if the fastpath succeeds, or -1 otherwise.
*/
static inline int
__mutex_fastpath_lock_retval(atomic_t *count)
{
if (unlikely(atomic_xchg(count, 0) != 1))
if (likely(atomic_xchg(count, -1) != 1))
return -1;
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.
*/
static inline void
__mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
{
if (unlikely(atomic_xchg(count, 1) != 0))
fail_fn(count);
}
#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;
}
return prev;
}
#endif
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