[PATCH] dma-fence: Bypass signaling annotation from dma_fence_is_signaled

[Tvrtko Ursulin]

On 09/06/2023 07:32, Christian König wrote:
> Am 08.06.23 um 16:30 schrieb Tvrtko Ursulin:
>> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>>
>> For dma_fence_is_signaled signaling critical path annotations are an
>> annoying cause of false positives when using dma_fence_is_signaled and
>> indirectly higher level helpers such as dma_resv_test_signaled etc.
>>
>> Drop the critical path annotation since the "is signaled" API does not
>> guarantee to ever change the signaled status anyway.
>>
>> We do that by adding a low level _dma_fence_signal helper and use it from
>> dma_fence_is_signaled.
> 
> I have been considering dropping the signaling from the 
> dma_fence_is_signaled() function altogether.
> 
> Doing this together with the spin locking we have in the dma_fence is 
> just utterly nonsense since the purpose of the external spinlock is to 
> keep the signaling in order while this here defeats that.
> 
> The quick check is ok I think, but signaling the dma_fence and issuing 
> the callbacks should always come from the interrupt handler.

What do you think is broken with regards to ordering with the current 
code? The unlocked fast check?

Regards,

Tvrtko

> 
> Christian.
> 
>>
>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>> Cc: Christian König <christian.koenig at amd.com>
>> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
>> ---
>>   drivers/dma-buf/dma-fence.c | 26 ++++++++++++++++++++------
>>   include/linux/dma-fence.h   |  3 ++-
>>   2 files changed, 22 insertions(+), 7 deletions(-)
>>
>> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
>> index f177c56269bb..ace1415f0566 100644
>> --- a/drivers/dma-buf/dma-fence.c
>> +++ b/drivers/dma-buf/dma-fence.c
>> @@ -444,6 +444,25 @@ int dma_fence_signal_locked(struct dma_fence *fence)
>>   }
>>   EXPORT_SYMBOL(dma_fence_signal_locked);
>> +/**
>> + * __dma_fence_signal - signal completion of a fence bypassing 
>> critical section annotation
>> + * @fence: the fence to signal
>> + *
>> + * This low-level helper should not be used by code external to 
>> dma-fence.h|c!
>> + */
>> +int _dma_fence_signal(struct dma_fence *fence)
>> +{
>> +    unsigned long flags;
>> +    int ret;
>> +
>> +    spin_lock_irqsave(fence->lock, flags);
>> +    ret = dma_fence_signal_timestamp_locked(fence, ktime_get());
>> +    spin_unlock_irqrestore(fence->lock, flags);
>> +
>> +    return ret;
>> +}
>> +EXPORT_SYMBOL(_dma_fence_signal);
>> +
>>   /**
>>    * dma_fence_signal - signal completion of a fence
>>    * @fence: the fence to signal
>> @@ -459,7 +478,6 @@ EXPORT_SYMBOL(dma_fence_signal_locked);
>>    */
>>   int dma_fence_signal(struct dma_fence *fence)
>>   {
>> -    unsigned long flags;
>>       int ret;
>>       bool tmp;
>> @@ -467,11 +485,7 @@ int dma_fence_signal(struct dma_fence *fence)
>>           return -EINVAL;
>>       tmp = dma_fence_begin_signalling();
>> -
>> -    spin_lock_irqsave(fence->lock, flags);
>> -    ret = dma_fence_signal_timestamp_locked(fence, ktime_get());
>> -    spin_unlock_irqrestore(fence->lock, flags);
>> -
>> +    ret = _dma_fence_signal(fence);
>>       dma_fence_end_signalling(tmp);
>>       return ret;
>> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
>> index d54b595a0fe0..d94768ad70e4 100644
>> --- a/include/linux/dma-fence.h
>> +++ b/include/linux/dma-fence.h
>> @@ -387,6 +387,7 @@ static inline void dma_fence_end_signalling(bool 
>> cookie) {}
>>   static inline void __dma_fence_might_wait(void) {}
>>   #endif
>> +int _dma_fence_signal(struct dma_fence *fence);
>>   int dma_fence_signal(struct dma_fence *fence);
>>   int dma_fence_signal_locked(struct dma_fence *fence);
>>   int dma_fence_signal_timestamp(struct dma_fence *fence, ktime_t 
>> timestamp);
>> @@ -452,7 +453,7 @@ dma_fence_is_signaled(struct dma_fence *fence)
>>           return true;
>>       if (fence->ops->signaled && fence->ops->signaled(fence)) {
>> -        dma_fence_signal(fence);
>> +        _dma_fence_signal(fence);
>>           return true;
>>       }
>