[PATCH 1/5] dma-buf: fix dma_resv_test_signaled test_all handling

[Christian König]

Am 11.06.21 um 16:47 schrieb Daniel Vetter:
> On Fri, Jun 11, 2021 at 02:02:57PM +0200, Christian König wrote:
>> As the name implies if testing all fences is requested we
>> should indeed test all fences and not skip the exclusive
>> one because we see shared ones.
>>
>> Signed-off-by: Christian König <christian.koenig at amd.com>
> Hm I thought we've had the rule that when both fences exist, then
> collectively the shared ones must signale no earlier than the exclusive
> one.
>
> That's at least the contract we've implemented in dma_resv.h. But I've
> also found a bunch of drivers who are a lot more yolo on this.
>
> I think there's a solid case here to just always take all the fences if we
> ask for all the shared ones, but if we go that way then I'd say
> - clear kerneldoc patch to really hammer this in (currently we're not good
>    at all in this regard)
> - going through drivers a bit to check for this (I have some of that done
>    already in my earlier series, need to respin it and send it out)
>
> But I'm kinda not seeing why this needs to be in this patch series here.

You mentioned that this is a problem in the last patch and if you ask me 
that's just a bug or at least very inconsistent.

See dma_resv_wait_timeout() always waits for all fences, including the 
exclusive one even if shared ones are present. But 
dma_resv_test_signaled() ignores the exclusive one if shared ones are 
present.

The only other driver I could find trying to make use of this is nouveau 
and I already provided a fix for this as well.

I just think that this is the more defensive approach to fix this and 
have at least the core functions consistent on the handling.

Christian.

> -Daniel
>
>> ---
>>   drivers/dma-buf/dma-resv.c | 33 ++++++++++++---------------------
>>   1 file changed, 12 insertions(+), 21 deletions(-)
>>
>> diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
>> index f26c71747d43..c66bfdde9454 100644
>> --- a/drivers/dma-buf/dma-resv.c
>> +++ b/drivers/dma-buf/dma-resv.c
>> @@ -615,25 +615,21 @@ static inline int dma_resv_test_signaled_single(struct dma_fence *passed_fence)
>>    */
>>   bool dma_resv_test_signaled(struct dma_resv *obj, bool test_all)
>>   {
>> -	unsigned int seq, shared_count;
>> +	struct dma_fence *fence;
>> +	unsigned int seq;
>>   	int ret;
>>   
>>   	rcu_read_lock();
>>   retry:
>>   	ret = true;
>> -	shared_count = 0;
>>   	seq = read_seqcount_begin(&obj->seq);
>>   
>>   	if (test_all) {
>>   		struct dma_resv_list *fobj = dma_resv_shared_list(obj);
>> -		unsigned int i;
>> -
>> -		if (fobj)
>> -			shared_count = fobj->shared_count;
>> +		unsigned int i, shared_count;
>>   
>> +		shared_count = fobj ? fobj->shared_count : 0;
>>   		for (i = 0; i < shared_count; ++i) {
>> -			struct dma_fence *fence;
>> -
>>   			fence = rcu_dereference(fobj->shared[i]);
>>   			ret = dma_resv_test_signaled_single(fence);
>>   			if (ret < 0)
>> @@ -641,24 +637,19 @@ bool dma_resv_test_signaled(struct dma_resv *obj, bool test_all)
>>   			else if (!ret)
>>   				break;
>>   		}
>> -
>> -		if (read_seqcount_retry(&obj->seq, seq))
>> -			goto retry;
>>   	}
>>   
>> -	if (!shared_count) {
>> -		struct dma_fence *fence_excl = dma_resv_excl_fence(obj);
>> -
>> -		if (fence_excl) {
>> -			ret = dma_resv_test_signaled_single(fence_excl);
>> -			if (ret < 0)
>> -				goto retry;
>> +	fence = dma_resv_excl_fence(obj);
>> +	if (fence) {
>> +		ret = dma_resv_test_signaled_single(fence);
>> +		if (ret < 0)
>> +			goto retry;
>>   
>> -			if (read_seqcount_retry(&obj->seq, seq))
>> -				goto retry;
>> -		}
>>   	}
>>   
>> +	if (read_seqcount_retry(&obj->seq, seq))
>> +		goto retry;
>> +
>>   	rcu_read_unlock();
>>   	return ret;
>>   }
>> -- 
>> 2.25.1
>>