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Date: Fri, 8 Jan 2016 10:41:51 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: bigeasy@linutronix.de, peterz@infradead.org, linux-kernel@vger.kernel.org,
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cc: linux-tip-commits@vger.kernel.org
Subject: Re: [tip:sched/core] sched/core: Reset task' s lockless wake-queues
 on fork()
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On Wed, 6 Jan 2016, tip-bot for Sebastian Andrzej Siewior wrote:

> Commit-ID:  093e5840ae76f1082633503964d035f40ed0216d
> Gitweb:     http://git.kernel.org/tip/093e5840ae76f1082633503964d035f40ed0216d
> Author:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> AuthorDate: Mon, 21 Dec 2015 18:17:10 +0100
> Committer:  Ingo Molnar <mingo@kernel.org>
> CommitDate: Wed, 6 Jan 2016 11:01:07 +0100
> 
> sched/core: Reset task's lockless wake-queues on fork()
> 
> In the following commit:
> 
>   7675104990ed ("sched: Implement lockless wake-queues")
> 
> we gained lockless wake-queues.
> 
> The -RT kernel managed to lockup itself with those. There could be multiple
> attempts for task X to enqueue it for a wakeup _even_ if task X is already
> running.
> 
> The reason is that task X could be runnable but not yet on CPU. The the
> task performing the wakeup did not leave the CPU it could performe
> multiple wakeups.
> 
> With the proper timming task X could be running and enqueued for a
> wakeup. If this happens while X is performing a fork() then its its
> child will have a !NULL `wake_q` member copied.
> 
> This is not a problem as long as the child task does not participate in
> lockless wakeups :)
>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: Davidlohr Bueso <dbueso@suse.de>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Fixes: 7675104990ed ("sched: Implement lockless wake-queues")

Shouldn't that go into stable?

Thanks,

	tglx