Message-ID: <534B9165.4000101@canonical.com>
Date:	Mon, 14 Apr 2014 09:42:29 +0200
From:	Maarten Lankhorst <maarten.lankhorst@...onical.com>
To:	Thomas Hellstrom <thellstrom@...are.com>
CC:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
	ccross@...gle.com, linux-media@...r.kernel.org
Subject: Re: [PATCH 2/2] [RFC v2 with seqcount] reservation: add suppport
 for read-only access using rcu

op 11-04-14 21:35, Thomas Hellstrom schreef:
> On 04/11/2014 08:09 PM, Maarten Lankhorst wrote:
>> op 11-04-14 12:11, Thomas Hellstrom schreef:
>>> On 04/11/2014 11:24 AM, Maarten Lankhorst wrote:
>>>> op 11-04-14 10:38, Thomas Hellstrom schreef:
>>>>> Hi, Maarten.
>>>>>
>>>>> Here I believe we encounter a lot of locking inconsistencies.
>>>>>
>>>>> First, it seems you're use a number of pointers as RCU pointers
>>>>> without
>>>>> annotating them as such and use the correct rcu
>>>>> macros when assigning those pointers.
>>>>>
>>>>> Some pointers (like the pointers in the shared fence list) are both
>>>>> used
>>>>> as RCU pointers (in dma_buf_poll()) for example,
>>>>> or considered protected by the seqlock
>>>>> (reservation_object_get_fences_rcu()), which I believe is OK, but then
>>>>> the pointers must
>>>>> be assigned using the correct rcu macros. In the memcpy in
>>>>> reservation_object_get_fences_rcu() we might get away with an
>>>>> ugly typecast, but with a verbose comment that the pointers are
>>>>> considered protected by the seqlock at that location.
>>>>>
>>>>> So I've updated (attached) the headers with proper __rcu annotation
>>>>> and
>>>>> locking comments according to how they are being used in the various
>>>>> reading functions.
>>>>> I believe if we want to get rid of this we need to validate those
>>>>> pointers using the seqlock as well.
>>>>> This will generate a lot of sparse warnings in those places needing
>>>>> rcu_dereference()
>>>>> rcu_assign_pointer()
>>>>> rcu_dereference_protected()
>>>>>
>>>>> With this I think we can get rid of all ACCESS_ONCE macros: It's not
>>>>> needed when the rcu_x() macros are used, and
>>>>> it's never needed for the members protected by the seqlock, (provided
>>>>> that the seq is tested). The only place where I think that's
>>>>> *not* the case is at the krealloc in
>>>>> reservation_object_get_fences_rcu().
>>>>>
>>>>> Also I have some more comments in the
>>>>> reservation_object_get_fences_rcu() function below:
>>>> I felt that the barriers needed for rcu were already provided by
>>>> checking the seqcount lock.
>>>> But looking at rcu_dereference makes it seem harmless to add it in
>>>> more places, it handles
>>>> the ACCESS_ONCE and barrier() for us.
>>> And it makes the code more maintainable, and helps sparse doing a lot of
>>> checking for us. I guess
>>> we can tolerate a couple of extra barriers for that.
>>>
>>>> We could probably get away with using RCU_INIT_POINTER on the writer
>>>> side,
>>>> because the smp_wmb is already done by arranging seqcount updates
>>>> correctly.
>>> Hmm. yes, probably. At least in the replace function. I think if we do
>>> it in other places, we should add comments as to where
>>> the smp_wmb() is located, for future reference.
>>>
>>>
>>> Also  I saw in a couple of places where you're checking the shared
>>> pointers, you're not checking for NULL pointers, which I guess may
>>> happen if shared_count and pointers are not in full sync?
>>>
>> No, because shared_count is protected with seqcount. I only allow
>> appending to the array, so when
>> shared_count is validated by seqcount it means that the
>> [0...shared_count) indexes are valid and non-null.
>> What could happen though is that the fence at a specific index is
>> updated with another one from the same
>> context, but that's harmless.
>>
> Hmm, doesn't attaching an exclusive fence clear all shared fence
> pointers from under a reader?
>
No, for that reason. It only resets shared_count to 0. This is harmless because the shared fence pointers are
still valid long enough because of RCU delayed deletion. fence_get_rcu will fail when the refcount has
dropped to zero. This is enough of a check to prevent errors, so there's no need to explicitly clear the fence
pointers.

~Maarten
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