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

[Christian König]

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.

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;
>   	}
>