linux-kernel - Re: [PATCH 1/8] lockdep: fix recursive read lock validation

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Date:	Tue, 29 Apr 2008 16:57:30 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Bart Van Assche <bart.vanassche@...il.com>
Cc:	ego@...ibm.com, linux-kernel@...r.kernel.org,
	Zdenek Kabelac <zdenek.kabelac@...il.com>,
	Oleg Nesterov <oleg@...sign.ru>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>
Subject: Re: [PATCH 1/8] lockdep: fix recursive read lock validation

On Tue, 2008-04-29 at 15:16 +0200, Bart Van Assche wrote:
> On Tue, Apr 29, 2008 at 2:57 PM, Gautham R Shenoy <ego@...ibm.com> wrote:
> > Subject: lockdep: fix recursive read lock validation
> >  This means that the following sequence is now invalid, whereas previously
> >  it was considered valid:
> >
> >   rlock(a); rlock(b); runlock(b); runlock(a)
> >   rlock(b); rlock(a);
> 
> Why are you marking this sequence as invalid ? Although it can be
> debated whether it is good programming practice to be inconsistent
> about the order of read-locking, the above sequence can't be involved
> in a deadlock.

Not for pure read locks, but when you add write locks to it, it does get
deadlocky. Lockdep does not keep separate chains for read and write
locks.

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