From 620a6fd185c084aa617c411f711533f01ea673c9 Mon Sep 17 00:00:00 2001 From: Ingo Molnar Date: Mon, 9 Jan 2006 15:59:17 -0800 Subject: [PATCH] mutex subsystem, add asm-generic/mutex-[dec|xchg|null].h implementations 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 Signed-off-by: Arjan van de Ven --- include/asm-generic/mutex-xchg.h | 117 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 117 insertions(+) create mode 100644 include/asm-generic/mutex-xchg.h (limited to 'include/asm-generic/mutex-xchg.h') diff --git a/include/asm-generic/mutex-xchg.h b/include/asm-generic/mutex-xchg.h new file mode 100644 index 00000000000..1d24f47e6c4 --- /dev/null +++ b/include/asm-generic/mutex-xchg.h @@ -0,0 +1,117 @@ +/* + * 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 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 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 + * 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 + * 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 -- cgit v1.2.3