Messages in this thread Patch in this message | | | Date | Fri, 27 Apr 2018 02:43:09 -0700 | From | tip-bot for Will Deacon <> | Subject | [tip:locking/core] locking/qspinlock: Use try_cmpxchg() instead of cmpxchg() when locking |
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Commit-ID: ae75d9089ff7095d1d1a12c3cd86b21d3eaf3b15 Gitweb: https://git.kernel.org/tip/ae75d9089ff7095d1d1a12c3cd86b21d3eaf3b15 Author: Will Deacon <will.deacon@arm.com> AuthorDate: Thu, 26 Apr 2018 11:34:26 +0100 Committer: Ingo Molnar <mingo@kernel.org> CommitDate: Fri, 27 Apr 2018 09:48:52 +0200
locking/qspinlock: Use try_cmpxchg() instead of cmpxchg() when locking
When reaching the head of an uncontended queue on the qspinlock slow-path, using a try_cmpxchg() instead of a cmpxchg() operation to transition the lock work to _Q_LOCKED_VAL generates slightly better code for x86 and pretty much identical code for arm64.
Reported-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Will Deacon <will.deacon@arm.com> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Waiman Long <longman@redhat.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: boqun.feng@gmail.com Cc: linux-arm-kernel@lists.infradead.org Cc: paulmck@linux.vnet.ibm.com Link: http://lkml.kernel.org/r/1524738868-31318-13-git-send-email-will.deacon@arm.com Signed-off-by: Ingo Molnar <mingo@kernel.org> --- kernel/locking/qspinlock.c | 19 +++++++++---------- 1 file changed, 9 insertions(+), 10 deletions(-)
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c index 956a12983bd0..46813185957b 100644 --- a/kernel/locking/qspinlock.c +++ b/kernel/locking/qspinlock.c @@ -467,16 +467,15 @@ locked: * Otherwise, we only need to grab the lock. */ - /* In the PV case we might already have _Q_LOCKED_VAL set */ - if ((val & _Q_TAIL_MASK) == tail) { - /* - * The atomic_cond_read_acquire() call above has provided the - * necessary acquire semantics required for locking. - */ - old = atomic_cmpxchg_relaxed(&lock->val, val, _Q_LOCKED_VAL); - if (old == val) - goto release; /* No contention */ - } + /* + * In the PV case we might already have _Q_LOCKED_VAL set. + * + * The atomic_cond_read_acquire() call above has provided the + * necessary acquire semantics required for locking. + */ + if (((val & _Q_TAIL_MASK) == tail) && + atomic_try_cmpxchg_relaxed(&lock->val, &val, _Q_LOCKED_VAL)) + goto release; /* No contention */ /* Either somebody is queued behind us or _Q_PENDING_VAL is set */ set_locked(lock);
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