From: Nick Piggin <npiggin@suse.de>
To: "Kristian Høgsberg" <krh@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
krh@bitplanet.net, eric@anholt.net,
Wang Chen <wangchen@cn.fujitsu.com>,
dri-devel@lists.sf.net, linux-kernel@vger.kernel.org,
Andrew Morton <akpm@linux-foundation.org>,
Hugh Dickins <hugh@veritas.com>
Subject: Re: [PATCH] drm: Take mmap_sem up front to avoid lock order violations.
Date: Thu, 19 Feb 2009 16:17:01 +0100 [thread overview]
Message-ID: <20090219151701.GG1747@wotan.suse.de> (raw)
In-Reply-To: <1235054980.31384.16.camel@gaara.bos.redhat.com>
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.
next prev parent reply other threads:[~2009-02-19 15:17 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-02-18 0:59 [PATCH] drm: Fix lock order reversal between mmap_sem and struct_mutex Eric Anholt
2009-02-18 8:02 ` Wang Chen
2009-02-18 16:38 ` [PATCH] drm: Take mmap_sem up front to avoid lock order violations krh
2009-02-19 9:19 ` Peter Zijlstra
2009-02-19 10:33 ` Peter Zijlstra
2009-02-19 14:49 ` Kristian Høgsberg
2009-02-19 15:17 ` Nick Piggin [this message]
2009-02-19 15:21 ` Kristian Høgsberg
2009-02-19 12:57 ` Nick Piggin
2009-02-21 2:33 ` Eric Anholt
2009-02-18 15:08 ` [PATCH] drm: Fix lock order reversal between mmap_sem and struct_mutex Peter Zijlstra
2009-02-19 21:02 ` Thomas Hellstrom
2009-02-19 22:26 ` Peter Zijlstra
2009-02-20 2:04 ` Eric Anholt
2009-02-20 7:36 ` Peter Zijlstra
2009-02-25 8:15 ` Eric Anholt
2009-02-25 8:54 ` Thomas Hellström
2009-02-25 9:07 ` Peter Zijlstra
2009-02-20 8:31 ` Thomas Hellstrom
2009-02-20 8:47 ` Peter Zijlstra
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