[Intel-gfx] [PATCH 2/2] drm/i915/execlists: Skip nested spinlock for validating pending

[Tvrtko Ursulin]

On 03/12/2019 11:53, Chris Wilson wrote:
> Only along the submission path can we guarantee that the locked request
> is indeed from a foreign engine, and so the nesting of engine/rq is
> permissible. On the submission tasklet (process_csb()), we may find
> ourselves competing with the normal nesting of rq/engine, invalidating
> our nesting. As we only use the spinlock for debug purposes, skip the
> debug if we cannot acquire the spinlock for safe validation - catching
> 99% of the bugs is better than causing a hard lockup.
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> ---
>   drivers/gpu/drm/i915/gt/intel_lrc.c | 7 +++----
>   1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> index 37ab9742abe7..b411e4ce6771 100644
> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> @@ -1300,7 +1300,6 @@ assert_pending_valid(const struct intel_engine_execlists *execlists,
>   	}
>   
>   	for (port = execlists->pending; (rq = *port); port++) {
> -		unsigned long flags;
>   		bool ok = true;
>   
>   		GEM_BUG_ON(!kref_read(&rq->fence.refcount));
> @@ -1315,8 +1314,8 @@ assert_pending_valid(const struct intel_engine_execlists *execlists,
>   		ce = rq->hw_context;
>   
>   		/* Hold tightly onto the lock to prevent concurrent retires! */
> -		spin_lock_irqsave_nested(&rq->lock, flags,
> -					 SINGLE_DEPTH_NESTING);
> +		if (!spin_trylock(&rq->lock))
> +			continue;
>   
>   		if (i915_request_completed(rq))
>   			goto unlock;
> @@ -1347,7 +1346,7 @@ assert_pending_valid(const struct intel_engine_execlists *execlists,
>   		}
>   
>   unlock:
> -		spin_unlock_irqrestore(&rq->lock, flags);
> +		spin_unlock(&rq->lock);
>   		if (!ok)
>   			return false;
>   	}
> 

With Fixes: and irqsave variant:

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>

Regards,

Tvrtko