From 0952e8f0e62456d7b6eb0abb3f71213b7630cf0f Mon Sep 17 00:00:00 2001 From: Will Deacon Date: Tue, 18 Dec 2018 23:10:46 +0100 Subject: locking/qspinlock: Kill cmpxchg() loop when claiming lock from head of queue commit c61da58d8a9ba9238250a548f00826eaf44af0f7 upstream. When a queued locker reaches the head of the queue, it claims the lock by setting _Q_LOCKED_VAL in the lockword. If there isn't contention, it must also clear the tail as part of this operation so that subsequent lockers can avoid taking the slowpath altogether. Currently this is expressed as a cmpxchg() loop that practically only runs up to two iterations. This is confusing to the reader and unhelpful to the compiler. Rewrite the cmpxchg() loop without the loop, so that a failed cmpxchg() implies that there is contention and we just need to write to _Q_LOCKED_VAL without considering the rest of the lockword. Signed-off-by: Will Deacon Acked-by: Peter Zijlstra (Intel) Acked-by: Waiman Long Cc: Linus Torvalds Cc: Thomas Gleixner Cc: boqun.feng@gmail.com Cc: linux-arm-kernel@lists.infradead.org Cc: paulmck@linux.vnet.ibm.com Link: http://lkml.kernel.org/r/1524738868-31318-7-git-send-email-will.deacon@arm.com Signed-off-by: Ingo Molnar Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Sasha Levin --- kernel/locking/qspinlock.c | 19 ++++++++----------- 1 file changed, 8 insertions(+), 11 deletions(-) (limited to 'kernel') diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c index e7ab99a1f438..ba5dc86a4d83 100644 --- a/kernel/locking/qspinlock.c +++ b/kernel/locking/qspinlock.c @@ -581,24 +581,21 @@ locked: * and nobody is pending, clear the tail code and grab the lock. * Otherwise, we only need to grab the lock. */ - for (;;) { - /* In the PV case we might already have _Q_LOCKED_VAL set */ - if ((val & _Q_TAIL_MASK) != tail || (val & _Q_PENDING_MASK)) { - set_locked(lock); - break; - } + + /* In the PV case we might already have _Q_LOCKED_VAL set */ + if ((val & _Q_TAIL_MASK) == tail) { /* * The smp_cond_load_acquire() call above has provided the - * necessary acquire semantics required for locking. At most - * two iterations of this loop may be ran. + * necessary acquire semantics required for locking. */ old = atomic_cmpxchg_relaxed(&lock->val, val, _Q_LOCKED_VAL); if (old == val) - goto release; /* No contention */ - - val = old; + goto release; /* No contention */ } + /* Either somebody is queued behind us or _Q_PENDING_VAL is set */ + set_locked(lock); + /* * contended path; wait for next if not observed yet, release. */ -- cgit v1.2.3