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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 12/14] locking/qspinlock: Use try_cmpxchg instead of cmpxchg when locking
Date: Thu, 26 Apr 2018 11:34:26 +0100	[thread overview]
Message-ID: <1524738868-31318-13-git-send-email-will.deacon@arm.com> (raw)
In-Reply-To: <1524738868-31318-1-git-send-email-will.deacon@arm.com>

When reaching the head of an uncontended queue on the qspinlock slow-path,
using a try_cmpxchg instead of a cmpxchg operation to transition the
lock work to _Q_LOCKED_VAL generates slightly better code for x86 and
pretty much identical code for arm64.

Cc: Ingo Molnar <mingo@kernel.org>
Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
---
 kernel/locking/qspinlock.c | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index fa5d2ab369f9..1e3ddc42135e 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -467,16 +467,15 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 	 * Otherwise, we only need to grab the lock.
 	 */
 
-	/* In the PV case we might already have _Q_LOCKED_VAL set */
-	if ((val & _Q_TAIL_MASK) == tail) {
-		/*
-		 * The atomic_cond_read_acquire() call above has provided the
-		 * necessary acquire semantics required for locking.
-		 */
-		old = atomic_cmpxchg_relaxed(&lock->val, val, _Q_LOCKED_VAL);
-		if (old == val)
-			goto release; /* No contention */
-	}
+	/*
+	 * In the PV case we might already have _Q_LOCKED_VAL set.
+	 *
+	 * The atomic_cond_read_acquire() call above has provided the
+	 * necessary acquire semantics required for locking.
+	 */
+	if (((val & _Q_TAIL_MASK) == tail) &&
+	    atomic_try_cmpxchg_relaxed(&lock->val, &val, _Q_LOCKED_VAL))
+		goto release; /* No contention */
 
 	/* Either somebody is queued behind us or _Q_PENDING_VAL is set */
 	set_locked(lock);
-- 
2.1.4

  parent reply	other threads:[~2018-04-26 10:35 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 ` [PATCH v3 06/14] locking/qspinlock: Kill cmpxchg loop when claiming lock from head of queue Will Deacon
2018-04-27  9:39   ` [tip:locking/core] locking/qspinlock: Kill cmpxchg() " 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 ` Will Deacon [this message]
2018-04-27  9:43   ` [tip:locking/core] locking/qspinlock: Use try_cmpxchg() instead of cmpxchg() when locking 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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