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Subject: Re: [PATCH 04/21] sched: Change the ttwu success details
From: Peter Zijlstra <peterz@infradead.org>
To: Chris Mason <chris.mason@oracle.com>
Cc: Frank Rowand <frank.rowand@am.sony.com>, Ingo Molnar <mingo@elte.hu>,
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        Jens Axboe <axboe@kernel.dk>, Yong Zhang <yong.zhang0@gmail.com>,
        linux-kernel@vger.kernel.org, Tejun Heo <tj@kernel.org>
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Date: Wed, 13 Apr 2011 13:06:39 +0200
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On Wed, 2011-04-13 at 12:48 +0200, Peter Zijlstra wrote:
> On Wed, 2011-04-13 at 11:23 +0200, Peter Zijlstra wrote:
> > and workqueue wakeups, and I doubt extra wakeups will cause lockups. 
> 
> Damn assumptions ;-)
> 
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -2447,7 +2447,7 @@ static inline void ttwu_post_activation(
>  	}
>  #endif
>  	/* if a worker is waking up, notify workqueue */
> -	if ((p->flags & PF_WQ_WORKER) && success)
> +	if (p->flags & PF_WQ_WORKER)
>  		wq_worker_waking_up(p, cpu_of(rq));
>  }
>  
> 
> Appears to be sufficient to cause the lockup, so somehow the whole
> workqueue stuff relies on the fact that waking a TASK_(UN)INTERRUPTIBLE
> task that hasn't been dequeued yet isn't a wakeup.
> 
> Tejun any quick clues as to why and how to cure this?
> 
> /me goes read that stuff

OK, so wq_worker_waking_up() does an atomic_inc() that wants to be
balanced against the atomic_dec() in wq_worker_sleeping(), which is only
called when we dequeue things.