linux-kernel - Re: [PATCH] drm: Fix lock order reversal between mmap_sem and struct

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Date:	Thu, 19 Feb 2009 23:26:12 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Thomas Hellstrom <thomas@...pmail.org>
Cc:	Eric Anholt <eric@...olt.net>, Wang Chen <wangchen@...fujitsu.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Ingo Molnar <mingo@...e.hu>, dri-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm: Fix lock order reversal between mmap_sem and
 struct_mutex.

On Thu, 2009-02-19 at 22:02 +0100, Thomas Hellstrom wrote:
>  
> It looks to me like the driver preferred locking order is
> 
> object_mutex (which happens to be the device global struct_mutex)
>   mmap_sem
>      offset_mutex.
> 
> So if one could avoid using the struct_mutex for object bookkeeping (A 
> separate lock) then
> vm_open() and vm_close() would adhere to that locking order as well, 
> simply by not taking the struct_mutex at all.
> 
> So only fault() remains, in which that locking order is reversed. 
> Personally I think the trylock ->reschedule->retry method with proper 
> commenting is a good solution. It will be the _only_ place where locking 
> order is reversed and it is done in a deadlock-safe manner. Note that 
> fault() doesn't really fail, but requests a retry from user-space with 
> rescheduling to give the process holding the struct_mutex time to 
> release it.

It doesn't do the reschedule -- need_resched() will check if the current
task was marked to be scheduled away, furthermore yield based locking
sucks chunks.

What's so very difficult about pulling the copy_*_user() out from under
the locks?

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