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/* * 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