From: Peter Zijlstra <peterz@infradead.org>
To: will.deacon@arm.com, mingo@kernel.org
Cc: linux-kernel@vger.kernel.org, longman@redhat.com,
andrea.parri@amarulasolutions.com, tglx@linutronix.de,
Peter Zijlstra <peterz@infradead.org>
Subject: [RFC][PATCH 1/3] locking/qspinlock: Re-order code
Date: Wed, 26 Sep 2018 13:01:18 +0200 [thread overview]
Message-ID: <20180926111307.401531130@infradead.org> (raw)
In-Reply-To: 20180926110117.405325143@infradead.org
[-- Attachment #1: peterz-qspinlock-opt-2.patch --]
[-- Type: text/plain, Size: 2298 bytes --]
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>
---
kernel/locking/qspinlock.c | 56 +++++++++++++++++++++------------------------
1 file changed, 27 insertions(+), 29 deletions(-)
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -330,39 +330,37 @@ void queued_spin_lock_slowpath(struct qs
* 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:[~2018-09-26 11:30 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-26 11:01 [RFC][PATCH 0/3] locking/qspinlock: Improve determinism for x86 Peter Zijlstra
2018-09-26 11:01 ` Peter Zijlstra [this message]
2018-10-01 17:17 ` [RFC][PATCH 1/3] locking/qspinlock: Re-order code Will Deacon
2018-09-26 11:01 ` [RFC][PATCH 2/3] locking/qspinlock: Rework some comments Peter Zijlstra
2018-10-01 17:17 ` Will Deacon
2018-10-01 19:10 ` Peter Zijlstra
2018-10-02 13:20 ` Will Deacon
2018-10-02 13:43 ` Peter Zijlstra
2018-09-26 11:01 ` [RFC][PATCH 3/3] locking/qspinlock: Optimize for x86 Peter Zijlstra
2018-09-26 16:30 ` Waiman Long
2018-09-26 17:54 ` Peter Zijlstra
2018-09-27 7:29 ` Peter Zijlstra
2018-09-26 20:52 ` Andrea Parri
2018-09-27 7:17 ` Peter Zijlstra
2018-09-27 7:47 ` Andrea Parri
2018-09-27 7:59 ` Peter Zijlstra
2018-09-27 8:13 ` Andrea Parri
2018-09-27 8:57 ` Peter Zijlstra
2018-09-27 12:16 ` David Laight
2018-10-01 17:17 ` Will Deacon
2018-10-01 20:00 ` Peter Zijlstra
2018-10-02 13:19 ` Will Deacon
2018-10-02 14:14 ` Peter Zijlstra
2018-10-02 12:31 ` Andrea Parri
2018-10-02 13:22 ` Will Deacon
2018-10-02 13:44 ` Andrea Parri
2018-09-26 15:01 ` [RFC][PATCH 0/3] locking/qspinlock: Improve determinism " Sebastian Andrzej Siewior
2018-09-26 15:08 ` Thomas Gleixner
2018-09-26 15:38 ` Sebastian Andrzej Siewior
2018-09-26 16:20 ` Waiman Long
2018-09-26 17:51 ` Peter Zijlstra
2018-09-26 23:21 ` Waiman Long
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