linux-kernel - Re: [PATCH 1/5] dma-buf: remove fallback for !CONFIG_DMA_SHARED

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Message-ID: <CAKMK7uGg5pbReAUA+cKWk-jyS3YwkUaZXE7MTcv9w7sk-4a10A@mail.gmail.com>
Date:	Wed, 3 Oct 2012 10:53:16 +0200
From:	Daniel Vetter <daniel@...ll.ch>
To:	Thomas Hellstrom <thellstrom@...are.com>
Cc:	Maarten Lankhorst <maarten.lankhorst@...onical.com>,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	linaro-mm-sig@...ts.linaro.org, sumit.semwal@...aro.org,
	linux-media@...r.kernel.org
Subject: Re: [PATCH 1/5] dma-buf: remove fallback for !CONFIG_DMA_SHARED_BUFFER

On Wed, Oct 3, 2012 at 10:37 AM, Thomas Hellstrom <thellstrom@...are.com> wrote:
>>> So if I understand you correctly, the reservation changes in TTM are
>>> motivated by the
>>> fact that otherwise, in the generic reservation code, lockdep can only be
>>> annotated for a trylock and not a waiting lock, when it *is* in fact a
>>> waiting lock.
>>>
>>> I'm completely unfamiliar with setting up lockdep annotations, but the
>>> only
>>> place a
>>> deadlock might occur is if the trylock fails and we do a
>>> wait_for_unreserve().
>>> Isn't it possible to annotate the call to wait_for_unreserve() just like
>>> an
>>> interruptible waiting lock
>>> (that is always interrupted, but at least any deadlock will be catched?).
>>
>> Hm, I have to admit that idea hasn't crossed my mind, but it's indeed
>> a hole in our current reservation lockdep annotations - since we're
>> blocking for the unreserve, other threads could potential block
>> waiting on us to release a lock we're holding already, resulting in a
>> deadlock.
>>
>> Since no other locking primitive that I know of has this
>> wait_for_unlocked interface, I don't know how we could map this in
>> lockdep. One idea is to grab the lock and release it again immediately
>> (only in the annotations, not the real lock ofc). But I need to check
>> the lockdep code to see whether that doesn't trip it up.
>
>
> I imagine doing the same as mutex_lock_interruptible() does in the
> interrupted path should work...

It simply calls the unlock lockdep annotation function if it breaks
out. So doing a lock/unlock cycle in wait_unreserve should do what we
want.

And to properly annotate the ttm reserve paths we could just add an
unconditional wait_unreserve call at the beginning like you suggested
(maybe with #ifdef CONFIG_PROVE_LOCKING in case ppl freak out about
the added atomic read in the uncontended case).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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