Subject: Re: [PATCH] Increase lockdep MAX_LOCK_DEPTH
From: Peter Zijlstra <peterz@infradead.org>
To: David Chinner <dgc@sgi.com>
Cc: Eric Sandeen <sandeen@sandeen.net>,
       linux-kernel Mailing List <linux-kernel@vger.kernel.org>,
       xfs-oss <xfs@oss.sgi.com>, Ingo Molnar <mingo@elte.hu>
In-Reply-To: <20070831150511.GA734179@sgi.com>
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	 <20070831150511.GA734179@sgi.com>
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Date: Fri, 31 Aug 2007 17:09:21 +0200
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On Sat, 2007-09-01 at 01:05 +1000, David Chinner wrote:

> > Trouble is, we'd like to have a sane upper bound on the amount of held
> > locks at any one time, obviously this is just wanting, because a lot of
> > lock chains also depend on the number of online cpus...
> 
> Sure - this is an obvious case where it is valid to take >30 locks at
> once in a single thread. In fact, worst case here we are taking twice this
> number of locks - we actually take 2 per inode (ilock and flock) so a
> full 32 inode cluster free would take >60 locks in the middle of this
> function and we should be busting this depth couter limit all the
> time. 

I think this started because jeffpc couldn't boot without XFS busting
lockdep :-)

> Do semaphores (the flush locks) contribute to the lock depth
> counters? 

No, alas, we cannot handle semaphores in lockdep. Semaphores don't have
a strict owner, hence we cannot track them. This is one of the reasons
to rid ourselves of semaphores - that and there are very few cases where
the actual semantics of semaphores are needed. Most of the times code
using semaphores can be expressed with either a mutex or a completion.

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