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locking/mutex: Optimize mutex_trylock() fast-path 

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>
This commit is contained in:
Peter Zijlstra 2016-06-01 20:58:15 +02:00 committed by Ingo Molnar
parent ddd0fa73c2
commit 6428671bae
7 changed files with 12 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
arch/ia64/include/asm/mutex.h
View File

@ -82,7 +82,7 @@ __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
static inline int
__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
{
if (cmpxchg_acq(count, 1, 0) == 1)
if (atomic_read(count) == 1 && cmpxchg_acq(count, 1, 0) == 1)
return 1;
return 0;
}

2
arch/powerpc/include/asm/mutex.h
View File

@ -124,7 +124,7 @@ __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
static inline int
__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
{
if (likely(__mutex_cmpxchg_lock(count, 1, 0) == 1))
if (likely(atomic_read(count) == 1 && __mutex_cmpxchg_lock(count, 1, 0) == 1))
return 1;
return 0;
}

2
arch/x86/include/asm/mutex_32.h
View File

@ -101,7 +101,7 @@ static inline int __mutex_fastpath_trylock(atomic_t *count,
int (*fail_fn)(atomic_t *))
{
/* cmpxchg because it never induces a false contention state. */
if (likely(atomic_cmpxchg(count, 1, 0) == 1))
if (likely(atomic_read(count) == 1 && atomic_cmpxchg(count, 1, 0) == 1))
return 1;
return 0;

6
arch/x86/include/asm/mutex_64.h
View File

@ -118,10 +118,10 @@ do { \
static inline int __mutex_fastpath_trylock(atomic_t *count,
int (*fail_fn)(atomic_t *))
{
if (likely(atomic_cmpxchg(count, 1, 0) == 1))
if (likely(atomic_read(count) == 1 && atomic_cmpxchg(count, 1, 0) == 1))
return 1;
else
return 0;
return 0;
}
#endif /* _ASM_X86_MUTEX_64_H */

2
include/asm-generic/mutex-dec.h
View File

@ -80,7 +80,7 @@ __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
static inline int
__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
{
if (likely(atomic_cmpxchg_acquire(count, 1, 0) == 1))
if (likely(atomic_read(count) == 1 && atomic_cmpxchg_acquire(count, 1, 0) == 1))
return 1;
return 0;
}

6
include/asm-generic/mutex-xchg.h
View File

@ -91,8 +91,12 @@ __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
static inline int
__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
{
int prev = atomic_xchg_acquire(count, 0);
int prev;
if (atomic_read(count) != 1)
return 0;
prev = atomic_xchg_acquire(count, 0);
if (unlikely(prev < 0)) {
/*
* The lock was marked contended so we must restore that

1
kernel/rcu/tree.c
View File

@ -3681,7 +3681,6 @@ static bool exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s) &&
(rnp == rnp_root ||
ULONG_CMP_LT(READ_ONCE(rnp_root->exp_seq_rq), s)) &&
!mutex_is_locked(&rsp->exp_mutex) &&
mutex_trylock(&rsp->exp_mutex))
goto fastpath;