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
next prev 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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