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6428671bae
A while back Viro posted a number of 'interesting' mutex_is_locked() users on IRC, one of those was RCU. RCU seems to use mutex_is_locked() to avoid doing mutex_trylock(), the regular load before modify pattern. While the use isn't wrong per se, its curious in that its needed at all, mutex_trylock() should be good enough on its own to avoid the pointless cacheline bounces. So fix those and remove the mutex_is_locked() (ab)use from RCU. Reported-by: Al Viro <viro@ZenIV.linux.org.uk> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Paul McKenney <paulmck@linux.vnet.ibm.com> Acked-by: Davidlohr Bueso <dave@stgolabs.net> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Waiman Long <Waiman.Long@hpe.com> Link: http://lkml.kernel.org/r/20160601185815.GW3190@twins.programming.kicks-ass.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
111 lines
3.3 KiB
C
111 lines
3.3 KiB
C
/*
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* Assembly implementation of the mutex fastpath, based on atomic
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* decrement/increment.
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*
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* started by Ingo Molnar:
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*
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* Copyright (C) 2004, 2005, 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
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*/
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#ifndef _ASM_X86_MUTEX_32_H
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#define _ASM_X86_MUTEX_32_H
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#include <asm/alternative.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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* @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 <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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#define __mutex_fastpath_lock(count, fail_fn) \
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do { \
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unsigned int dummy; \
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\
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typecheck(atomic_t *, count); \
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typecheck_fn(void (*)(atomic_t *), fail_fn); \
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\
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asm volatile(LOCK_PREFIX " decl (%%eax)\n" \
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" jns 1f \n" \
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" call " #fail_fn "\n" \
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"1:\n" \
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: "=a" (dummy) \
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: "a" (count) \
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: "memory", "ecx", "edx"); \
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} while (0)
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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 __mutex_fastpath_lock_retval(atomic_t *count)
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{
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if (unlikely(atomic_dec_return(count) < 0))
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return -1;
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else
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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 <fail_fn>.
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* In the failure case, this function is allowed to either set the value
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* to 1, or to set it to a value lower than 1.
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*
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* If the implementation sets it to a value of lower than 1, 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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#define __mutex_fastpath_unlock(count, fail_fn) \
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do { \
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unsigned int dummy; \
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\
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typecheck(atomic_t *, count); \
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typecheck_fn(void (*)(atomic_t *), fail_fn); \
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\
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asm volatile(LOCK_PREFIX " incl (%%eax)\n" \
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" jg 1f\n" \
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" call " #fail_fn "\n" \
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"1:\n" \
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: "=a" (dummy) \
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: "a" (count) \
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: "memory", "ecx", "edx"); \
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} while (0)
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#define __mutex_slowpath_needs_to_unlock() 1
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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: fallback function
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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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static inline int __mutex_fastpath_trylock(atomic_t *count,
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int (*fail_fn)(atomic_t *))
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{
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/* cmpxchg because it never induces a false contention state. */
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if (likely(atomic_read(count) == 1 && atomic_cmpxchg(count, 1, 0) == 1))
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return 1;
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return 0;
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}
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#endif /* _ASM_X86_MUTEX_32_H */
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