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120 lines
3.8 KiB
120 lines
3.8 KiB
/* |
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* include/asm-generic/mutex-xchg.h |
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* |
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* Generic implementation of the mutex fastpath, based on xchg(). |
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* |
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* NOTE: An xchg based implementation might be less optimal than an atomic |
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* decrement/increment based implementation. If your architecture |
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* has a reasonable atomic dec/inc then you should probably use |
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* asm-generic/mutex-dec.h instead, or you could open-code an |
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* optimized version in asm/mutex.h. |
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*/ |
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#ifndef _ASM_GENERIC_MUTEX_XCHG_H |
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#define _ASM_GENERIC_MUTEX_XCHG_H |
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/** |
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* __mutex_fastpath_lock - try to take the lock by moving the count |
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* from 1 to a 0 value |
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* @count: pointer of type atomic_t |
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* @fail_fn: function to call if the original value was not 1 |
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* |
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* Change the count from 1 to a value lower than 1, and call <fail_fn> if it |
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* wasn't 1 originally. This function MUST leave the value lower than 1 |
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* even when the "1" assertion wasn't true. |
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*/ |
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static inline void |
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__mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *)) |
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{ |
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if (unlikely(atomic_xchg(count, 0) != 1)) |
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/* |
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* We failed to acquire the lock, so mark it contended |
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* to ensure that any waiting tasks are woken up by the |
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* unlock slow path. |
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*/ |
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if (likely(atomic_xchg_acquire(count, -1) != 1)) |
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fail_fn(count); |
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} |
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/** |
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* __mutex_fastpath_lock_retval - try to take the lock by moving the count |
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* from 1 to a 0 value |
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* @count: pointer of type atomic_t |
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* |
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* Change the count from 1 to a value lower than 1. This function returns 0 |
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* if the fastpath succeeds, or -1 otherwise. |
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*/ |
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static inline int |
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__mutex_fastpath_lock_retval(atomic_t *count) |
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{ |
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if (unlikely(atomic_xchg_acquire(count, 0) != 1)) |
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if (likely(atomic_xchg(count, -1) != 1)) |
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return -1; |
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return 0; |
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} |
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/** |
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* __mutex_fastpath_unlock - try to promote the mutex from 0 to 1 |
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* @count: pointer of type atomic_t |
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* @fail_fn: function to call if the original value was not 0 |
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* |
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* try to promote the mutex from 0 to 1. if it wasn't 0, call <function> |
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* In the failure case, this function is allowed to either set the value to |
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* 1, or to set it to a value lower than one. |
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* If the implementation sets it to a value of lower than one, the |
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* __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs |
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* to return 0 otherwise. |
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*/ |
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static inline void |
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__mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *)) |
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{ |
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if (unlikely(atomic_xchg_release(count, 1) != 0)) |
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fail_fn(count); |
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} |
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#define __mutex_slowpath_needs_to_unlock() 0 |
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/** |
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* __mutex_fastpath_trylock - try to acquire the mutex, without waiting |
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* |
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* @count: pointer of type atomic_t |
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* @fail_fn: spinlock based trylock implementation |
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* |
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* Change the count from 1 to a value lower than 1, and return 0 (failure) |
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* if it wasn't 1 originally, or return 1 (success) otherwise. This function |
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* MUST leave the value lower than 1 even when the "1" assertion wasn't true. |
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* Additionally, if the value was < 0 originally, this function must not leave |
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* it to 0 on failure. |
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* |
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* If the architecture has no effective trylock variant, it should call the |
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* <fail_fn> spinlock-based trylock variant unconditionally. |
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*/ |
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static inline int |
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__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *)) |
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{ |
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int prev; |
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if (atomic_read(count) != 1) |
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return 0; |
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prev = atomic_xchg_acquire(count, 0); |
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if (unlikely(prev < 0)) { |
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/* |
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* The lock was marked contended so we must restore that |
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* state. If while doing so we get back a prev value of 1 |
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* then we just own it. |
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* |
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* [ In the rare case of the mutex going to 1, to 0, to -1 |
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* and then back to 0 in this few-instructions window, |
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* this has the potential to trigger the slowpath for the |
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* owner's unlock path needlessly, but that's not a problem |
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* in practice. ] |
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*/ |
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prev = atomic_xchg_acquire(count, prev); |
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if (prev < 0) |
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prev = 0; |
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} |
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return prev; |
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} |
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#endif
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