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

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Message-ID: <20090130134935.GC17401@elte.hu>
Date:	Fri, 30 Jan 2009 14:49:35 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	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.


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Fri, 30 Jan 2009 16:33:53 +1030 Rusty Russell <rusty@...tcorp.com.au> wrote:
> 
> > On Thursday 29 January 2009 12:42:05 Andrew Morton wrote:
> > > 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)?
> > 
> > Just buggy.  What random thread was it mugging?  If there's any path 
> > where it's not a kthread, what if userspace does the same thing at the 
> > same time? We risk running on the wrong cpu, *then* overriding 
> > userspace when we restore it.
> 
> hm, Ok, not unficable but not pleasant.
> 
> > In general these cpumask games are a bad idea.
> 
> So we still don't have any non-buggy proposal.

Current upstream code is not pretty (due to the extra workqueue) but not 
buggy either. You'd be right to point out that it is easy to insert a bug 
into it and thus it's not pleasant (more of a workaround than a real fix) 
but if it's outright buggy then please talk up.

	Ingo
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