4.9-stable review patch. If anyone has any objections, please let me know.
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commit 53bf57fab7321fb42b703056a4c80fc9d986d170 upstream.
Flip the branch condition after atomic_fetch_or_acquire(_Q_PENDING_VAL) such that we loose the indent. This also result in a more natural code flow IMO.
Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Acked-by: Will Deacon will.deacon@arm.com Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Peter Zijlstra peterz@infradead.org Cc: Thomas Gleixner tglx@linutronix.de Cc: andrea.parri@amarulasolutions.com Cc: longman@redhat.com Link: https://lkml.kernel.org/r/20181003130257.156322446@infradead.org 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 | 54 ++++++++++++++++++-------------------- 1 file changed, 26 insertions(+), 28 deletions(-)
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c index ba5dc86a4d83..f493a4fce624 100644 --- a/kernel/locking/qspinlock.c +++ b/kernel/locking/qspinlock.c @@ -440,38 +440,36 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) * 0,0,1 -> 0,1,1 ; pending */ val = atomic_fetch_or_acquire(_Q_PENDING_VAL, &lock->val); - if (!(val & ~_Q_LOCKED_MASK)) { - /* - * We're pending, wait for the owner to go away. - * - * *,1,1 -> *,1,0 - * - * this wait loop must be a load-acquire such that we match the - * store-release that clears the locked bit and create lock - * sequentiality; this is because not all - * clear_pending_set_locked() implementations imply full - * barriers. - */ - if (val & _Q_LOCKED_MASK) { - smp_cond_load_acquire(&lock->val.counter, - !(VAL & _Q_LOCKED_MASK)); - } - - /* - * take ownership and clear the pending bit. - * - * *,1,0 -> *,0,1 - */ - clear_pending_set_locked(lock); - return; + /* + * If we observe any contention; undo and queue. + */ + if (unlikely(val & ~_Q_LOCKED_MASK)) { + if (!(val & _Q_PENDING_MASK)) + clear_pending(lock); + goto queue; }
/* - * If pending was clear but there are waiters in the queue, then - * we need to undo our setting of pending before we queue ourselves. + * We're pending, wait for the owner to go away. + * + * 0,1,1 -> 0,1,0 + * + * this wait loop must be a load-acquire such that we match the + * store-release that clears the locked bit and create lock + * sequentiality; this is because not all + * clear_pending_set_locked() implementations imply full + * barriers. + */ + if (val & _Q_LOCKED_MASK) + smp_cond_load_acquire(&lock->val.counter, !(VAL & _Q_LOCKED_MASK)); + + /* + * take ownership and clear the pending bit. + * + * 0,1,0 -> 0,0,1 */ - if (!(val & _Q_PENDING_MASK)) - clear_pending(lock); + clear_pending_set_locked(lock); + return;
/* * End of pending bit optimistic spinning and beginning of MCS