From: tip-bot for Peter Zijlstra <tipbot@zytor.com>
To: linux-tip-commits@vger.kernel.org
Cc: mingo@kernel.org, peterz@infradead.org,
torvalds@linux-foundation.org, hpa@zytor.com, tglx@linutronix.de,
linux-kernel@vger.kernel.org, will.deacon@arm.com
Subject: [tip:locking/core] locking/qspinlock: Re-order code
Date: Tue, 16 Oct 2018 09:04:25 -0700 [thread overview]
Message-ID: <tip-53bf57fab7321fb42b703056a4c80fc9d986d170@git.kernel.org> (raw)
In-Reply-To: <20181003130257.156322446@infradead.org>
Commit-ID: 53bf57fab7321fb42b703056a4c80fc9d986d170
Gitweb: https://git.kernel.org/tip/53bf57fab7321fb42b703056a4c80fc9d986d170
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Wed, 26 Sep 2018 13:01:18 +0200
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 16 Oct 2018 17:33:53 +0200
locking/qspinlock: Re-order code
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>
---
kernel/locking/qspinlock.c | 56 ++++++++++++++++++++++------------------------
1 file changed, 27 insertions(+), 29 deletions(-)
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index bfaeb05123ff..ec343276f975 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -330,39 +330,37 @@ 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) {
- atomic_cond_read_acquire(&lock->val,
- !(VAL & _Q_LOCKED_MASK));
- }
-
- /*
- * take ownership and clear the pending bit.
- *
- * *,1,0 -> *,0,1
- */
- clear_pending_set_locked(lock);
- qstat_inc(qstat_lock_pending, true);
- 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)
+ atomic_cond_read_acquire(&lock->val, !(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);
+ qstat_inc(qstat_lock_pending, true);
+ return;
/*
* End of pending bit optimistic spinning and beginning of MCS
next prev parent reply other threads:[~2018-10-16 16:04 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-03 13:02 [PATCH v2 0/4] locking/qspinlock, x86: Improve determinism for x86 Peter Zijlstra
2018-10-03 13:02 ` [PATCH v2 1/4] locking/qspinlock: Re-order code Peter Zijlstra
2018-10-03 13:02 ` [PATCH v2 2/4] locking/qspinlock: Rework some comments Peter Zijlstra
2018-10-10 16:13 ` Will Deacon
2018-10-03 13:03 ` [PATCH v2 3/4] x86/asm: Simplify GEN_*_RMWcc() macros Peter Zijlstra
2018-10-04 9:18 ` Peter Zijlstra
2018-10-16 16:05 ` [tip:locking/core] x86/asm: 'Simplify' " tip-bot for Peter Zijlstra
2018-10-03 13:03 ` [PATCH v2 4/4] locking/qspinlock, x86: Provide liveness guarantee Peter Zijlstra
2018-10-10 16:12 ` Will Deacon
2018-10-12 9:22 ` Will Deacon
2018-10-16 16:06 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2018-10-16 16:04 ` tip-bot for Peter Zijlstra [this message]
2018-10-16 16:04 ` [tip:locking/core] locking/qspinlock: Rework some comments tip-bot for Peter Zijlstra
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