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Subject: Re: [PATCH] drm: Take mmap_sem up front to avoid lock order
 violations.
From: Kristian =?ISO-8859-1?Q?H=F8gsberg?= <krh@redhat.com>
To: Nick Piggin <npiggin@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>, krh@bitplanet.net, eric@anholt.net,
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Date: Thu, 19 Feb 2009 10:21:16 -0500
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On Thu, 2009-02-19 at 16:17 +0100, Nick Piggin wrote:
> On Thu, Feb 19, 2009 at 09:49:40AM -0500, Kristian Høgsberg wrote:
> > 
> > > Secondly, mmap_sem is not a recursive lock (very few kernel locks are,
> > > and we generally frown upon recursive locking schemes), this means that
> > > the fault handler still cannot function properly.
> > 
> > I understand, but we take it twice only as a read lock, so that should
> > work, right?  We prevent the deadlock the lockdep validator warned about
> > and as far as I can see, the patch doesn't introduce a new one.  But
> > other than that I agree with the frowning on recursive locking, it's too
> > often used to paper over badly thought out locking.
> 
> It doesn't work. rwsems are fair (otherwise there is terrible starvation
> properties), so if another process does an interleaved down_write, then
> the 2nd down_read will block until the down_write is serviced.

Ooh, right, yes of course, ouch.

thanks,
Kristian