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Date: Wed, 28 Jan 2009 18:12:05 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Rusty Russell <rusty@rustcorp.com.au>
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
Subject: Re: [PATCH 2/3] work_on_cpu: Use our own workqueue.
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On Thu, 29 Jan 2009 12:13:32 +1030 Rusty Russell <rusty@rustcorp.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)?