Commit-ID: ae75d9089ff7095d1d1a12c3cd86b21d3eaf3b15 Gitweb: https://git.kernel.org/tip/ae75d9089ff7095d1d1a12c3cd86b21d3eaf3b15 Author: Will Deacon <[email protected]> AuthorDate: Thu, 26 Apr 2018 11:34:26 +0100 Committer: Ingo Molnar <[email protected]> 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 <[email protected]> Signed-off-by: Will Deacon <[email protected]> Acked-by: Peter Zijlstra (Intel) <[email protected]> Acked-by: Waiman Long <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Thomas Gleixner <[email protected]> Cc: [email protected] Cc: [email protected] Cc: [email protected] Link: http://lkml.kernel.org/r/[email protected] Signed-off-by: Ingo Molnar <[email protected]> --- 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);
