From: Davidlohr Bueso <dave@stgolabs.net>
To: Peter Zijlstra <peterz@infradead.org>
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 14:08:06 -0700 [thread overview]
Message-ID: <20150914210806.GG19736@linux-q0g1.site> (raw)
In-Reply-To: <20150914123241.GR18489@twins.programming.kicks-ass.net>
On Mon, 14 Sep 2015, Peter Zijlstra wrote:
>On Mon, Sep 14, 2015 at 12:37:23AM -0700, Davidlohr Bueso wrote:
>> /*
>> + * 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.
Well, I was just taking advantage of removing the upper barrier. Considering
that the formal semantics, you are right that we need not actual acquire per-se
(ie for node->next) but instead merely ensure a barrier in wake_q_add(). This is
kind of why I had hinted of going full _relaxed(). We could also rephrase the
comment, something like:
* Use ACQUIRE semantics to add the next pointer, such that
* wake_q_add() implies a full barrier. This pairs with the
* write barrier implied by the wakeup in wake_up_list().
*/
What do you think?
next prev parent reply other threads:[~2015-09-14 21:08 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
2015-09-14 21:08 ` Davidlohr Bueso [this message]
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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