linux-kernel - Re: workqueue deadlock

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Message-Id: <20061209114723.138b6e89.akpm@osdl.org>
Date:	Sat, 9 Dec 2006 11:47:23 -0800
From:	Andrew Morton <akpm@...l.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	vatsa@...ibm.com, Bjorn Helgaas <bjorn.helgaas@...com>,
	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 Sat, 9 Dec 2006 11:26:52 +0100
Ingo Molnar <mingo@...e.hu> wrote:

> 
> * Andrew Morton <akpm@...l.org> wrote:
> 
> > > > +		if (cpu != -1)
> > > > +			mutex_lock(&workqueue_mutex);
> > > 
> > > events/4 thread itself wanting the same mutex above?
> > 
> > Could do, not sure.  I'm planning on converting all the locking around 
> > here to preempt_disable() though.
> 
> please at least use an owner-recursive per-CPU lock,

a wot?

> not a naked 
> preempt_disable()! The concurrency rules for data structures changed via 
> preempt_disable() are quite hard to sort out after the fact. 
> (preempt_disable() is too opaque,

preempt_disable() is the preferred way of holding off cpu hotplug.

> it doesnt attach data structure to 
> critical section, like normal locks do.)

the data structure is the CPU, and its per-cpu data.  And cpu_online_map.
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