From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <longman@redhat.com>
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 17:06:27 +0100 [thread overview]
Message-ID: <20181129160627.GU2131@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <5598cd71-c3c8-d6ef-eb30-777cf901a2ef@redhat.com>
On Thu, Nov 29, 2018 at 10:21:58AM -0500, Waiman Long wrote:
> >> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> >> index 09b1800..50d9af6 100644
> >> --- a/kernel/locking/rwsem-xadd.c
> >> +++ b/kernel/locking/rwsem-xadd.c
> >> @@ -198,15 +198,22 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
> >> woken++;
> >> tsk = waiter->task;
> >>
> >> - wake_q_add(wake_q, tsk);
> >> + get_task_struct(tsk);
> >> list_del(&waiter->list);
> >> /*
> >> - * Ensure that the last operation is setting the reader
> >> + * Ensure calling get_task_struct() before setting the reader
> >> * waiter to nil such that rwsem_down_read_failed() cannot
> >> * race with do_exit() by always holding a reference count
> >> * to the task to wakeup.
> >> */
> >> smp_store_release(&waiter->task, NULL);
> >> + /*
> >> + * Ensure issuing the wakeup (either by us or someone else)
> >> + * after setting the reader waiter to nil.
> >> + */
> >> + wake_q_add(wake_q, tsk);
> >> + /* wake_q_add() already take the task ref */
> >> + put_task_struct(tsk);
> >> }
> >>
> >> adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
>
> I doubt putting wake_q_add() after clearing waiter->task can really fix
Why; at that point we know the wakeup will happen after, which is all we
require.
next prev parent reply other threads:[~2018-11-29 16:06 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 [this message]
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
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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