From: Waiman Long <longman@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Yongji Xie <elohimes@gmail.com>,
mingo@redhat.com, will.deacon@arm.com,
linux-kernel@vger.kernel.org, xieyongji@baidu.com,
zhangyu31@baidu.com, liuqi16@baidu.com, yuanlinsi01@baidu.com,
nixun@baidu.com, lilin24@baidu.com,
Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [RFC] locking/rwsem: Avoid issuing wakeup before setting the reader waiter to nil
Date: Thu, 29 Nov 2018 13:26:34 -0500 [thread overview]
Message-ID: <729ceddb-dd9a-ec2a-f74e-03fa4d7e65e8@redhat.com> (raw)
In-Reply-To: <20181129180828.GA11650@hirez.programming.kicks-ass.net>
On 11/29/2018 01:08 PM, Peter Zijlstra wrote:
> On Thu, Nov 29, 2018 at 06:27:00PM +0100, Peter Zijlstra wrote:
>> wake_up_q() should, per the barriers in wake_up_process, ensure that if
>> wake_a_add() fails, there will be a wakeup of that task after that
>> point.
>>
>> So if we put wake_up_q() at the location where wake_up_process() should
>> be, it should all work.
>>
>> The bug in question is that it can happen at any time after
>> wake_q_add(), not necessarily at wake_up_q().
> Hmm, I think we're missing a barrier in wake_q_add(); when cmpxchg()
> fails we still need an smp_mb().
>
> Something like so.
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 3d87a28da378..69def558edf6 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -400,6 +400,13 @@ void wake_q_add(struct wake_q_head *head, struct task_struct *task)
> {
> struct wake_q_node *node = &task->wake_q;
>
> + /*
> + * Ensure, that when the below cmpxchg() fails, the corresponding
> + * wake_up_q() will observe our prior state.
> + *
> + * Pairs with the smp_mb() from wake_up_q()'s wake_up_process().
> + */
> + smp_mb();
> /*
> * 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
> @@ -408,7 +415,7 @@ void wake_q_add(struct wake_q_head *head, struct task_struct *task)
> * This cmpxchg() executes a full barrier, which pairs with the full
> * barrier executed by the wakeup in wake_up_q().
> */
> - if (cmpxchg(&node->next, NULL, WAKE_Q_TAIL))
> + if (cmpxchg_relaxed(&node->next, NULL, WAKE_Q_TAIL))
> return;
>
> get_task_struct(task);
That can be costly for x86 which will now have 2 locked instructions.
Should we introduce a kind of special cmpxchg (e.g. cmpxchg_mb) that
will guarantee a memory barrier whether the operation fails or not?
Cheers,
Longman
next prev parent reply other threads:[~2018-11-29 18:26 UTC|newest]
Thread overview: 50+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-29 12:50 [RFC] locking/rwsem: Avoid issuing wakeup before setting the reader waiter to nil Yongji Xie
2018-11-29 13:12 ` Peter Zijlstra
2018-11-29 13:44 ` Peter Zijlstra
2018-11-29 14:02 ` Yongji Xie
2018-11-29 18:43 ` Davidlohr Bueso
2018-11-29 18:49 ` Waiman Long
2018-11-29 15:21 ` Waiman Long
2018-11-29 15:29 ` Waiman Long
2018-11-29 16:06 ` Peter Zijlstra
2018-11-29 17:02 ` Waiman Long
2018-11-29 17:27 ` Peter Zijlstra
2018-11-29 17:58 ` Waiman Long
2018-11-29 18:13 ` Peter Zijlstra
2018-11-29 18:17 ` Davidlohr Bueso
2018-11-29 18:08 ` Peter Zijlstra
2018-11-29 18:26 ` Waiman Long [this message]
2018-11-29 18:31 ` Will Deacon
2018-11-29 18:34 ` Waiman Long
2018-11-29 22:05 ` Peter Zijlstra
2018-11-30 9:34 ` 答复: " Liu,Qi(ACU-T1)
2018-11-30 14:15 ` Peter Zijlstra
2018-11-29 21:30 ` Davidlohr Bueso
2018-11-29 21:34 ` Davidlohr Bueso
2018-11-29 22:17 ` Peter Zijlstra
2018-11-30 9:30 ` Andrea Parri
2018-12-03 5:31 ` [PATCH -tip] kernel/sched,wake_q: Branch predict wake_q_add() cmpxchg Davidlohr Bueso
2018-12-03 16:10 ` Waiman Long
2019-01-21 11:28 ` [tip:locking/core] sched/wake_q: Add branch prediction hint to " tip-bot for Davidlohr Bueso
2018-12-10 15:12 ` [RFC] locking/rwsem: Avoid issuing wakeup before setting the reader waiter to nil Yongji Xie
2018-12-17 11:37 ` Peter Zijlstra
2018-12-17 13:12 ` Yongji Xie
2019-01-07 14:35 ` Waiman Long
2019-01-07 15:31 ` Peter Zijlstra
2019-01-07 15:35 ` Waiman Long
2018-12-17 20:53 ` Davidlohr Bueso
2018-12-18 13:10 ` Peter Zijlstra
2018-12-18 13:14 ` Peter Zijlstra
2018-12-18 17:27 ` Davidlohr Bueso
2018-12-18 18:54 ` [PATCH v2] sched/wake_q: Reduce reference counting for special users Davidlohr Bueso
2018-12-18 19:17 ` Waiman Long
2018-12-18 19:30 ` Davidlohr Bueso
2018-12-18 19:39 ` Davidlohr Bueso
2018-12-18 19:53 ` [PATCH v4] " Davidlohr Bueso
2018-12-18 20:35 ` Waiman Long
2019-01-21 16:02 ` Davidlohr Bueso
2019-01-22 8:55 ` Peter Zijlstra
2019-02-04 8:57 ` [tip:locking/core] " tip-bot for Davidlohr Bueso
2019-02-07 19:30 ` Davidlohr Bueso
2019-02-12 14:14 ` Daniel Vacek
2019-01-21 11:28 ` [tip:locking/core] locking/rwsem: Fix (possible) missed wakeup tip-bot for Xie Yongji
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