From: Davidlohr Bueso <dave@stgolabs.net>
To: Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Davidlohr Bueso <dave@stgolabs.net>,
linux-kernel@vger.kernel.org, Davidlohr Bueso <dbueso@suse.de>
Subject: [PATCH -tip 2/3] sched/wake_q: Relax to acquire semantics
Date: Mon, 14 Sep 2015 00:37:23 -0700 [thread overview]
Message-ID: <1442216244-4409-2-git-send-email-dave@stgolabs.net> (raw)
In-Reply-To: <1442216244-4409-1-git-send-email-dave@stgolabs.net>
The barrier parings for wake-queues are very straightforward, and thus
we can ease the barrier requirements, for archs that support it, for
wake_q_add by relying on acquire semantics. As such, (i) we keep the
pairing structure/logic and (ii) users, such as mqueues, can continue to
rely on a full barrier after the successful [Rmw].
[Another alternative could be to just not try at all and fully downgrade
to cmpxchg_relaxed() and rely on users to enable their own synchronization.
But controlling this ourselves makes me sleep better at night.]
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
kernel/sched/core.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 6ab415a..7567603 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -523,14 +523,14 @@ void wake_q_add(struct wake_q_head *head, struct task_struct *task)
struct wake_q_node *node = &task->wake_q;
/*
- * Atomically grab the task, if ->wake_q is !nil already it means
- * its already queued (either by us or someone else) and will get the
- * wakeup due to that.
+ * Atomically grab the task. If ->wake_q is non-nil (failed cmpxchg)
+ * then the task is already queued (by us or someone else) and will
+ * get the wakeup due to that.
*
- * This cmpxchg() implies a full barrier, which pairs with the write
- * barrier implied by the wakeup in wake_up_list().
+ * Use acquire semantics to add the next pointer, which pairs with the
+ * write barrier implied by the wakeup in wake_up_list().
*/
- if (cmpxchg(&node->next, NULL, WAKE_Q_TAIL))
+ if (cmpxchg_acquire(&node->next, NULL, WAKE_Q_TAIL))
return;
get_task_struct(task);
--
2.1.4
next prev parent reply other threads:[~2015-09-14 7:37 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-09-14 7:37 [PATCH -tip 1/3] locking/qrwlock: Rename ->lock to ->wait_lock Davidlohr Bueso
2015-09-14 7:37 ` Davidlohr Bueso [this message]
2015-09-14 12:32 ` [PATCH -tip 2/3] sched/wake_q: Relax to acquire semantics Peter Zijlstra
2015-09-14 21:08 ` Davidlohr Bueso
2015-09-15 9:49 ` Peter Zijlstra
2015-09-15 9:55 ` Peter Zijlstra
2015-09-15 12:41 ` Paul E. McKenney
2015-09-15 12:48 ` Peter Zijlstra
2015-09-15 14:09 ` Paul E. McKenney
2015-09-15 14:14 ` Peter Zijlstra
2015-09-15 15:34 ` Paul E. McKenney
2015-09-15 16:30 ` Peter Zijlstra
2015-09-15 17:09 ` Paul E. McKenney
2015-09-18 21:41 ` Paul E. McKenney
2015-09-21 9:22 ` Martin Schwidefsky
2015-09-22 10:27 ` Martin Schwidefsky
2015-09-22 12:23 ` Boqun Feng
2015-09-22 12:51 ` Martin Schwidefsky
2015-09-22 13:29 ` Boqun Feng
2015-09-22 14:33 ` Martin Schwidefsky
2015-09-22 15:28 ` Paul E. McKenney
2015-09-23 6:43 ` Martin Schwidefsky
2015-09-25 21:30 ` Paul E. McKenney
2015-09-15 19:49 ` Davidlohr Bueso
2015-09-16 9:01 ` Peter Zijlstra
2015-09-14 7:37 ` [PATCH -tip 3/3] locking/osq: Relax atomic semantics Davidlohr Bueso
2015-09-18 8:50 ` [tip:locking/core] " tip-bot for Davidlohr Bueso
2015-09-18 8:50 ` [tip:locking/core] locking/qrwlock: Rename ->lock to ->wait_lock tip-bot for Davidlohr Bueso
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