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Message-ID: <20160602132724.GG5231@linux.vnet.ibm.com>
Date: Thu, 2 Jun 2016 06:27:24 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...nel.org>,
Davidlohr Bueso <dave@...olabs.net>,
Waiman Long <Waiman.Long@....com>, viro@...IV.linux.org.uk,
linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] mutex: Optimize mutex_trylock() fast-path
On Wed, Jun 01, 2016 at 08:58:15PM +0200, Peter Zijlstra wrote:
>
> 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.
Definitely -- the old versions of mutex_trylock() looked to take ownership
of the cacheline, which most definitely is not what you want if you have
a bunch of CPUs doing expedited grace periods concurrently.
> 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.
Given the changes below that push the mutex_is_locked() check into
mutex_trylock():
Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Or I could pull the changes into -rcu if you prefer.
Thanx, Paul
> Reported-by: Al Viro <viro@...IV.linux.org.uk>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
> arch/ia64/include/asm/mutex.h | 2 +-
> arch/powerpc/include/asm/mutex.h | 2 +-
> arch/x86/include/asm/mutex_32.h | 2 +-
> arch/x86/include/asm/mutex_64.h | 6 +++---
> include/asm-generic/mutex-dec.h | 2 +-
> include/asm-generic/mutex-xchg.h | 6 +++++-
> kernel/rcu/tree.c | 1 -
> 7 files changed, 12 insertions(+), 9 deletions(-)
>
> diff --git a/arch/ia64/include/asm/mutex.h b/arch/ia64/include/asm/mutex.h
> index f41e66d65e31..28cb819e0ff9 100644
> --- a/arch/ia64/include/asm/mutex.h
> +++ b/arch/ia64/include/asm/mutex.h
> @@ -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;
> }
> diff --git a/arch/powerpc/include/asm/mutex.h b/arch/powerpc/include/asm/mutex.h
> index 127ab23e1f6c..078155fa1189 100644
> --- a/arch/powerpc/include/asm/mutex.h
> +++ b/arch/powerpc/include/asm/mutex.h
> @@ -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;
> }
> diff --git a/arch/x86/include/asm/mutex_32.h b/arch/x86/include/asm/mutex_32.h
> index 85e6cda45a02..e9355a84fc67 100644
> --- a/arch/x86/include/asm/mutex_32.h
> +++ b/arch/x86/include/asm/mutex_32.h
> @@ -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;
> diff --git a/arch/x86/include/asm/mutex_64.h b/arch/x86/include/asm/mutex_64.h
> index 07537a44216e..d9850758464e 100644
> --- a/arch/x86/include/asm/mutex_64.h
> +++ b/arch/x86/include/asm/mutex_64.h
> @@ -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 */
> diff --git a/include/asm-generic/mutex-dec.h b/include/asm-generic/mutex-dec.h
> index fd694cfd678a..c54829d3de37 100644
> --- a/include/asm-generic/mutex-dec.h
> +++ b/include/asm-generic/mutex-dec.h
> @@ -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;
> }
> diff --git a/include/asm-generic/mutex-xchg.h b/include/asm-generic/mutex-xchg.h
> index a6b4a7bd6ac9..3269ec4e195f 100644
> --- a/include/asm-generic/mutex-xchg.h
> +++ b/include/asm-generic/mutex-xchg.h
> @@ -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
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index c7f1bc4f817c..b7326893221f 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -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;
>
>
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