linux-kernel - Re: _cpu_down deadlock [was Re: 2.6.19-rc1-mm1]

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Message-Id: <1160635872.3000.399.camel@laptopd505.fenrus.org>
Date:	Thu, 12 Oct 2006 08:51:12 +0200
From:	Arjan van de Ven <arjan@...radead.org>
To:	Neil Brown <neilb@...e.de>
Cc:	Andrew Morton <akpm@...l.org>,
	Michal Piotrowski <michal.k.k.piotrowski@...il.com>,
	Pavel Machek <pavel@....cz>, "Rafael J. Wysocki" <rjw@...k.pl>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	rusty@...tcorp.com.au
Subject: Re: _cpu_down deadlock [was Re: 2.6.19-rc1-mm1]

On Thu, 2006-10-12 at 09:46 +1000, Neil Brown wrote:
> On Wednesday October 11, akpm@...l.org wrote:
> > > 
> > > So A waits on B and C, C waits on B, B waits on A.
> > > Deadlock.
> > 
> > Except the entire operation is serialised by the the two top-level callers
> > (cpu_up() and cpu_down()) taking mutex_lock(&cpu_add_remove_lock).  Can
> > lockdep be taught about that?
> 
> So you are saying that even though we have locking sequences
>   A -> B  and B -> A,
> that cannot - in this case - cause a deadlock as both sequences only
> ever happen under a third exclusive lock C,
> So when lockdep records a lock-dependency A -> B, it should also
> record a list of locks that are *always* held when that dependency
> occurs.

in that case... why are A and B there *at all* ?


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