From: Peter Zijlstra <peterz@infradead.org>
To: Yongji Xie <elohimes@gmail.com>
Cc: 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>,
Waiman Long <longman@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [RFC] locking/rwsem: Avoid issuing wakeup before setting the reader waiter to nil
Date: Thu, 29 Nov 2018 14:44:49 +0100 [thread overview]
Message-ID: <20181129134449.GH2149@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20181129131232.GN2131@hirez.programming.kicks-ass.net>
On Thu, Nov 29, 2018 at 02:12:32PM +0100, Peter Zijlstra wrote:
>
> +Cc davidlohr and waiman
> Urgh; so the case where the cmpxchg() fails because it already has a
> wakeup in progress, which then 'violates' our expectation of when the
> wakeup happens.
>
> Yes, I think this is real, and worse, I think we need to go audit all
> wake_q_add() users and document this behaviour.
>
> In the ideal case we'd delay the actual wakeup to the last wake_up_q(),
> but I don't think we can easily fix that.
See commit: 1d0dcb3ad9d3 ("futex: Implement lockless wakeups"), I think
that introduces the exact same bug.
Something like the below perhaps, altough this pattern seems to want a
wake_a_add() variant that already assumes get_task_struct().
diff --git a/kernel/futex.c b/kernel/futex.c
index f423f9b6577e..d14971f6ed3d 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1387,11 +1387,7 @@ static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q)
if (WARN(q->pi_state || q->rt_waiter, "refusing to wake PI futex\n"))
return;
- /*
- * Queue the task for later wakeup for after we've released
- * the hb->lock. wake_q_add() grabs reference to p.
- */
- wake_q_add(wake_q, p);
+ get_task_struct(p);
__unqueue_futex(q);
/*
* The waiting task can free the futex_q as soon as q->lock_ptr = NULL
@@ -1401,6 +1397,13 @@ static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q)
* plist_del in __unqueue_futex().
*/
smp_store_release(&q->lock_ptr, NULL);
+
+ /*
+ * Queue the task for later wakeup for after we've released
+ * the hb->lock. wake_q_add() grabs reference to p.
+ */
+ wake_q_add(wake_q, p);
+ put_task_struct(p);
}
/*
next prev parent reply other threads:[~2018-11-29 13:45 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 [this message]
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
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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