From: Peter Zijlstra <peterz@infradead.org>
To: Davidlohr Bueso <dave@stgolabs.net>
Cc: Ingo Molnar <mingo@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
linux-kernel@vger.kernel.org, Davidlohr Bueso <dbueso@suse.de>
Subject: Re: [PATCH -tip 2/3] sched/wake_q: Relax to acquire semantics
Date: Mon, 14 Sep 2015 14:32:41 +0200 [thread overview]
Message-ID: <20150914123241.GR18489@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <1442216244-4409-2-git-send-email-dave@stgolabs.net>
On Mon, Sep 14, 2015 at 12:37:23AM -0700, Davidlohr Bueso wrote:
> 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].
> 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 non-nil (failed cmpxchg)
> + * then the task is already queued (by us or someone else) and will
> + * get the wakeup due to that.
> *
> + * Use acquire semantics to add the next pointer, which pairs with the
> + * write barrier implied by the wakeup in wake_up_list().
> */
> + if (cmpxchg_acquire(&node->next, NULL, WAKE_Q_TAIL))
> return;
>
> get_task_struct(task);
I'm not seeing a _why_ on the acquire semantics. Not saying the patch is
wrong, just saying I want words on why acquire is correct.
next prev parent reply other threads:[~2015-09-14 12:32 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 ` [PATCH -tip 2/3] sched/wake_q: Relax to acquire semantics Davidlohr Bueso
2015-09-14 12:32 ` Peter Zijlstra [this message]
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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