linux-kernel - Re: lockdep problem conversion semaphore->mutex (dev->sem)

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Message-Id: <1197147016.6353.53.camel@lappy>
Date:	Sat, 08 Dec 2007 21:50:16 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Remy Bohmer <linux@...mer.net>
Cc:	Daniel Walker <dwalker@...sta.com>, Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Dave Chinner <dgc@....com>
Subject: Re: lockdep problem conversion semaphore->mutex (dev->sem)

On Sat, 2007-12-08 at 21:33 +0100, Remy Bohmer wrote:

> Which problems? I did not see any special things, it looked rather
> straight forward. What have I overlooked?

On suspend it locks the whole device tree, this means it has 'unbounded'
nesting and holds an 'unbounded' number of locks. Neither things are
easy to annotate (remember that mutex_lock_nested can handle up to 8
nestings and current->held_locks has a max of 30).

In fact, converting this will be the hardest part, it would require
reworking the locking and introduction of a hard limit on the device
tree depth - this might upset some people, but I suspect that 16 or 24
should be deep enough for pretty much anything. Of course, if people
prove me wrong, I'll have to reconsider. The up-side of the locking
scheme I'm thinking of will be that locking the whole tree will only
take 'depth' number of opterations vs the total number of tree elements.

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