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- atomic operations which both modify the variable and return something imply full smp memory barriers before and after the memory operations involved (failing atomic_cmpxchg, atomic_add_unless, etc don't imply a barrier because they don't modify the target). See Documentation/atomic_ops.txt. So remove extra barriers and branches. - All architectures support atomic_cmpxchg. This has no relation to __HAVE_ARCH_CMPXCHG. We can just take the atomic_cmpxchg path unconditionally This reduces a simple single threaded fastpath lock+unlock test from 590 cycles to 203 cycles on a ppc970 system. Signed-off-by: Nick Piggin <npiggin@suse.de> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
111 lines
3.7 KiB
C
111 lines
3.7 KiB
C
/*
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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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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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* @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 returns 0 if the fastpath succeeds,
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* or anything the slow path function returns
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*/
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static inline int
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__mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *))
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{
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if (unlikely(atomic_xchg(count, 0) != 1))
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return fail_fn(count);
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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(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 = atomic_xchg(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(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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