Re: [Intel-gfx] [PATCH 01/20] drm/i915: Skip barriers inside waits

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 27/02/2020 08:57, Chris Wilson wrote:
> Attaching to the i915_active barrier is a two stage process, and a flush
> is only effective when the barrier is activation. Thus it is possible
> for us to see a barrier, and attempt to flush, only for our flush to
> have no effect. As such, before attempting to activate signaling on the
> fence we need to double check it is a fence!
> 
> Fixes: d13a31770077 ("drm/i915: Flush idle barriers when waiting")
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> ---
>   drivers/gpu/drm/i915/i915_active.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c
> index 0b12d5023800..7b3d6c12ad61 100644
> --- a/drivers/gpu/drm/i915/i915_active.c
> +++ b/drivers/gpu/drm/i915/i915_active.c
> @@ -453,6 +453,9 @@ static void enable_signaling(struct i915_active_fence *active)
>   {
>   	struct dma_fence *fence;
>   
> +	if (unlikely(is_barrier(active)))
> +		return;
> +
>   	fence = i915_active_fence_get(active);
>   	if (!fence)
>   		return;
> 

So that smp_rmb() is not really effective, I mean the race is wider than 
that. I was worried about that.. now I need to figure out where it 
starts and where it ends (the race).

Regards,

Tvrtko
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