linux-kernel - Re: [PATCH 04/21] sched: Change the ttwu success details

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Date:	Wed, 13 Apr 2011 12:48:23 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	Frank Rowand <frank.rowand@...sony.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <efault@....de>,
	Oleg Nesterov <oleg@...hat.com>, Paul Turner <pjt@...gle.com>,
	Jens Axboe <axboe@...nel.dk>,
	Yong Zhang <yong.zhang0@...il.com>,
	linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 04/21] sched: Change the ttwu success details

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

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