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From: Rusty Russell <rusty@rustcorp.com.au>
To: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 2/3] work_on_cpu: Use our own workqueue.
Date: Thu, 29 Jan 2009 12:13:32 +1030
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Cc: Mike Travis <travis@sgi.com>, Ingo Molnar <mingo@redhat.com>,
       Dave Jones <davej@redhat.com>, cpufreq@vger.kernel.org,
       linux-kernel@vger.kernel.org
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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.

Thanks,
Rusty.