linux-kernel - Re: workqueue deadlock

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Date:	Fri, 8 Dec 2006 08:23:01 +0530
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	Andrew Morton <akpm@...l.org>
Cc:	Bjorn Helgaas <bjorn.helgaas@...com>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org, Myron Stowe <myron.stowe@...com>,
	Jens Axboe <axboe@...nel.dk>, Dipankar <dipankar@...ibm.com>,
	Gautham shenoy <ego@...ibm.com>
Subject: Re: workqueue deadlock

On Thu, Dec 07, 2006 at 11:37:00AM -0800, Andrew Morton wrote:
> -static void flush_cpu_workqueue(struct cpu_workqueue_struct *cwq)
> +/*
> + * If cpu == -1 it's a single-threaded workqueue and the caller does not hold
> + * workqueue_mutex
> + */
> +static void flush_cpu_workqueue(struct cpu_workqueue_struct *cwq, int cpu)

Lets say @cpu = 4

>  {
>  	if (cwq->thread == current) {
>  		/*
>  		 * Probably keventd trying to flush its own queue. So simply run
>  		 * it by hand rather than deadlocking.
>  		 */
> +		if (cpu != -1)
> +			mutex_unlock(&workqueue_mutex);

Lets say we release the workqueue mutex here (events/4 is trying to
flush its own workqueue). Immediately another CPU takes this mutex 
(in CPU_DOWN_PREPARE) and brings down CPU4. In CPU_DEAD handling we now wait 
on events/4 thread to exit (cleanup_workqueue_thread).

Couldnt this wait deadlock on :

>  		run_workqueue(cwq);

> +		if (cpu != -1)
> +			mutex_lock(&workqueue_mutex);

events/4 thread itself wanting the same mutex above?

What am I missing?


-- 
Regards,
vatsa
-
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