Date:	Wed, 03 Oct 2012 10:35:43 +0200
From:	Thomas Hellstrom <thellstrom@...are.com>
To:	Maarten Lankhorst <maarten.lankhorst@...onical.com>
CC:	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, Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH 1/5] dma-buf: remove fallback for !CONFIG_DMA_SHARED_BUFFER

On 10/03/2012 09:57 AM, Maarten Lankhorst wrote:
> Hey,
>
> Op 03-10-12 09:45, Thomas Hellstrom schreef:
>> On 10/02/2012 10:03 AM, Daniel Vetter wrote:
>>> On Tue, Oct 02, 2012 at 08:46:32AM +0200, Thomas Hellstrom wrote:
>>>> On 10/01/2012 11:47 AM, Maarten Lankhorst wrote:
>>>>> I was doing a evil hack where I 'released' lru_lock to lockdep before doing the annotation
>>>>> for a blocking acquire, and left trylock annotations as they were. This made lockdep do the
>>>>> right thing.
>>>> I've never looked into how lockdep works. Is this something that can
>>>> be done permanently or just for testing
>>>> purposes? Although not related to this, is it possible to do
>>>> something similar to the trylock reversal in the
>>>> fault() code where mmap_sem() and reserve() change order using a
>>>> reserve trylock?
>>> lockdep just requires a bunch of annotations, is a compile-time configure
>>> option CONFIG_PROVE_LOCKING and if disabled, has zero overhead. And it's
>>> rather awesome in detected deadlocks and handling crazy locking schemes
>>> correctly:
>>> - correctly handles trylocks
>>> - correctly handles nested locking (i.e. grabbing a global lock, then
>>>     grabbing subordinate locks in an unordered sequence since the global
>>>     lock ensures that no deadlocks can happen).
>>> - any kinds of inversions with special contexts like hardirq, softirq
>>> - same for page-reclaim, i.e. it will yell if you could (potentially)
>>>     deadlock because your shrinker grabs a lock that you hold while calling
>>>     kmalloc.
>>> - there are special annotates for various subsystems, e.g. to check for
>>>     del_timer_sync vs. locks held by that timer. Or the console_lock
>>>     annotations I've just recently submitted.
>>> - all that with a really flexible set of annotation primitives that afaics
>>>     should work for almost any insane locking scheme. The fact that Maarten
>>>     could come up with proper reservation annotations without any changes to
>>>     lockdep testifies this (he only had to fix a tiny thing to make it a bit
>>>     more strict in a corner case).
>>>
>>> In short I think it's made of awesome. The only downside is that it lacks
>>> documentation, you have to read the code to understand it :(
>>>
>>> The reason I've suggested to Maarten to abolish the trylock_reservation
>>> within the lru_lock is that in that way lockdep only ever sees the
>>> trylock, and hence is less strict about complainig about deadlocks. But
>>> semantically it's an unconditional reserve. Maarten had some horrible
>>> hacks that leaked the lockdep annotations out of the new reservation code,
>>> which allowed ttm to be properly annotated.  But those also reduced the
>>> usefulness for any other users of the reservation code, and so Maarten
>>> looked into whether he could remove that trylock dance in ttm.
>>>
>>> Imo having excellent lockdep support for cross-device reservations is a
>>> requirment, and ending up with less strict annotations for either ttm
>>> based drivers or other drivers is not good. And imo the ugly layering that
>>> Maarten had in his first proof-of-concept also indicates that something is
>>> amiss in the design.
>>>
>>>
>> 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?).
> That would not find all bugs, lockdep is meant to find even theoretical bugs, so annotating it as a
> waiting lock makes more sense. Otherwise lockdep will only barf when the initial trylock fails.

Really, starting a waiting reserve with a call to wait_for_unreserve() 
if CONFIG_LOCKDEP is set
shouldn't be that bad :)? That would catch also the the theoretical errors.
In fact, it should suffice with annotating for such a call?

/Thomas

> ~Maarten
>

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