linux-kernel - Re: [PATCH] drm: Take mmap_sem up front to avoid lock order violations.

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Message-ID: <20090219151701.GG1747@wotan.suse.de>
Date:	Thu, 19 Feb 2009 16:17:01 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Kristian Høgsberg <krh@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, krh@...planet.net,
	eric@...olt.net, Wang Chen <wangchen@...fujitsu.com>,
	dri-devel@...ts.sf.net, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hugh@...itas.com>
Subject: Re: [PATCH] drm: Take mmap_sem up front to avoid lock order violations.

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.
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