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From: Will Deacon <will.deacon@arm.com>
To: linux-kernel@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org, peterz@infradead.org,
	mingo@kernel.org, boqun.feng@gmail.com,
	paulmck@linux.vnet.ibm.com, longman@redhat.com,
	will.deacon@arm.com
Subject: [PATCH v3 06/14] locking/qspinlock: Kill cmpxchg loop when claiming lock from head of queue
Date: Thu, 26 Apr 2018 11:34:20 +0100	[thread overview]
Message-ID: <1524738868-31318-7-git-send-email-will.deacon@arm.com> (raw)
In-Reply-To: <1524738868-31318-1-git-send-email-will.deacon@arm.com>

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.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
---
 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 ad94b7def005..e42f50d69ed6 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -465,24 +465,21 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 	 * 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.
 	 */
-- 
2.1.4

  parent reply	other threads:[~2018-04-26 10:37 UTC|newest]

Thread overview: 41+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-26 10:34 [PATCH v3 00/14] kernel/locking: qspinlock improvements Will Deacon
2018-04-26 10:34 ` [PATCH v3 01/14] barriers: Introduce smp_cond_load_relaxed and atomic_cond_read_relaxed Will Deacon
2018-04-27  9:36   ` [tip:locking/core] locking/barriers: Introduce smp_cond_load_relaxed() and atomic_cond_read_relaxed() tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 02/14] locking/qspinlock: Merge struct __qspinlock into struct qspinlock Will Deacon
2018-04-27  9:37   ` [tip:locking/core] locking/qspinlock: Merge 'struct __qspinlock' into 'struct qspinlock' tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 03/14] locking/qspinlock: Bound spinning on pending->locked transition in slowpath Will Deacon
2018-04-27  9:37   ` [tip:locking/core] " tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 04/14] locking/qspinlock/x86: Increase _Q_PENDING_LOOPS upper bound Will Deacon
2018-04-27  9:38   ` [tip:locking/core] " tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 05/14] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath Will Deacon
2018-04-26 15:53   ` Peter Zijlstra
2018-04-26 16:55     ` Will Deacon
2018-04-28 12:45       ` Peter Zijlstra
2018-04-30  8:53         ` Will Deacon
2018-04-26 20:16   ` Waiman Long
2018-04-27 10:16     ` Will Deacon
2018-04-27 11:01       ` [tip:locking/core] locking/qspinlock: Remove duplicate clear_pending() function from PV code tip-bot for Will Deacon
2018-04-27 13:09       ` [PATCH v3 05/14] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath Waiman Long
2018-04-27  9:39   ` [tip:locking/core] locking/qspinlock: Remove unbounded cmpxchg() " tip-bot for Will Deacon
2018-04-26 10:34 ` Will Deacon [this message]
2018-04-27  9:39   ` [tip:locking/core] locking/qspinlock: Kill cmpxchg() loop when claiming lock from head of queue tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 07/14] locking/qspinlock: Use atomic_cond_read_acquire Will Deacon
2018-04-27  9:40   ` [tip:locking/core] locking/qspinlock: Use atomic_cond_read_acquire() tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 08/14] locking/mcs: Use smp_cond_load_acquire() in mcs spin loop Will Deacon
2018-04-27  9:40   ` [tip:locking/core] locking/mcs: Use smp_cond_load_acquire() in MCS " tip-bot for Jason Low
2018-04-26 10:34 ` [PATCH v3 09/14] locking/qspinlock: Use smp_cond_load_relaxed to wait for next node Will Deacon
2018-04-27  9:41   ` [tip:locking/core] locking/qspinlock: Use smp_cond_load_relaxed() " tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 10/14] locking/qspinlock: Make queued_spin_unlock use smp_store_release Will Deacon
2018-04-27  9:42   ` [tip:locking/core] locking/qspinlock: Use smp_store_release() in queued_spin_unlock() tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 11/14] locking/qspinlock: Elide back-to-back RELEASE operations with smp_wmb() Will Deacon
2018-04-27  9:42   ` [tip:locking/core] " tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 12/14] locking/qspinlock: Use try_cmpxchg instead of cmpxchg when locking Will Deacon
2018-04-27  9:43   ` [tip:locking/core] locking/qspinlock: Use try_cmpxchg() instead of cmpxchg() " tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 13/14] locking/qspinlock: Add stat tracking for pending vs slowpath Will Deacon
2018-04-27  9:43   ` [tip:locking/core] locking/qspinlock: Add stat tracking for pending vs. slowpath tip-bot for Waiman Long
2018-04-26 10:34 ` [PATCH v3 14/14] MAINTAINERS: Add myself as a co-maintainer for LOCKING PRIMITIVES Will Deacon
2018-04-26 15:55   ` Peter Zijlstra
2018-04-27  9:44   ` [tip:locking/core] MAINTAINERS: Add myself as a co-maintainer for the locking subsystem tip-bot for Will Deacon
2018-04-26 15:54 ` [PATCH v3 00/14] kernel/locking: qspinlock improvements Peter Zijlstra
2018-04-27  9:33   ` Ingo Molnar
2018-04-26 20:18 ` Waiman Long

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