Re: [PATCH 2/2] [RFC v2 with seqcount] reservation: add suppport for read-only access using rcu

[Maarten Lankhorst <maarten.lankhorst@canonical.com>]

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.

~Maarten