linux-kernel - Re: [PATCH 2/3] work_on_cpu: Use our own workqueue.

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Message-Id: <20090128181205.3b15fa4a.akpm@linux-foundation.org>
Date:	Wed, 28 Jan 2009 18:12:05 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Mike Travis <travis@....com>, Ingo Molnar <mingo@...hat.com>,
	Dave Jones <davej@...hat.com>, cpufreq@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] work_on_cpu: Use our own workqueue.

On Thu, 29 Jan 2009 12:13:32 +1030 Rusty Russell <rusty@...tcorp.com.au> wrote:

> On Thursday 29 January 2009 06:14:40 Andrew Morton wrote:
> > It's vulnerable to the same deadlock, I think?  Suppose we have:
> ...
> > - A calls work_on_cpu() and takes woc_mutex.
> > 
> > - Before function_which_takes_L() has started to execute, task B takes L
> >   then calls work_on_cpu() and task B blocks on woc_mutex.
> > 
> > - Now function_which_takes_L() runs, and blocks on L
> 
> Agreed, but now it's a fairly simple case.  Both sides have to take lock L, and both have to call work_on_cpu.
> 
> Workqueues are more generic and widespread, and an amazing amount of stuff gets called from them.  That's why I felt uncomfortable with removing the one known problematic caller.
> 

hm.  it's a bit of a timebomb.

y'know, the original way in which acpi-cpufreq did this is starting to
look attractive.  Migrate self to that CPU then just call the dang
function.  Slow, but no deadlocks (I think)?


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