4.9-stable review patch. If anyone has any objections, please let me know.
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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 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/1524738868-31318-7-git-send-email-will.deacon@arm.c... Signed-off-by: Ingo Molnar mingo@kernel.org Signed-off-by: Sebastian Andrzej Siewior bigeasy@linutronix.de Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/locking/qspinlock.c | 19 ++++++++----------- 1 file changed, 8 insertions(+), 11 deletions(-)
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. */