Re: [PATCH] drm/i915: Always double check for a missed interrupt for new bottom halves

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

On 05/07/16 12:29, Chris Wilson wrote:
> After assigning ourselves as the new bottom-half, we must perform a
> cursory check to prevent a missed interrupt.  Either we miss the interrupt
> whilst programming the hardware, or if there was a previous waiter (for
> a later seqno) they may be woken instead of us (due to the inherent race
> in the unlocked read of b->tasklet in the irq handler) and so we miss the
> wake up.
>
> Spotted-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> ---
>   drivers/gpu/drm/i915/intel_breadcrumbs.c | 16 +++++++++++-----
>   1 file changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_breadcrumbs.c b/drivers/gpu/drm/i915/intel_breadcrumbs.c
> index 009d6e1..6fcbb52 100644
> --- a/drivers/gpu/drm/i915/intel_breadcrumbs.c
> +++ b/drivers/gpu/drm/i915/intel_breadcrumbs.c
> @@ -65,7 +65,7 @@ static void irq_disable(struct intel_engine_cs *engine)
>   	engine->irq_posted = false;
>   }
>
> -static bool __intel_breadcrumbs_enable_irq(struct intel_breadcrumbs *b)
> +static void __intel_breadcrumbs_enable_irq(struct intel_breadcrumbs *b)
>   {
>   	struct intel_engine_cs *engine =
>   		container_of(b, struct intel_engine_cs, breadcrumbs);
> @@ -73,7 +73,7 @@ static bool __intel_breadcrumbs_enable_irq(struct intel_breadcrumbs *b)
>
>   	assert_spin_locked(&b->lock);
>   	if (b->rpm_wakelock)
> -		return false;
> +		return;
>
>   	/* Since we are waiting on a request, the GPU should be busy
>   	 * and should have its own rpm reference. For completeness,
> @@ -93,8 +93,6 @@ static bool __intel_breadcrumbs_enable_irq(struct intel_breadcrumbs *b)
>   	if (!b->irq_enabled ||
>   	    test_bit(engine->id, &i915->gpu_error.missed_irq_rings))
>   		mod_timer(&b->fake_irq, jiffies + 1);
> -
> -	return engine->irq_posted;
>   }
>
>   static void __intel_breadcrumbs_disable_irq(struct intel_breadcrumbs *b)
> @@ -233,7 +231,15 @@ static bool __intel_engine_add_wait(struct intel_engine_cs *engine,
>   		GEM_BUG_ON(rb_first(&b->waiters) != &wait->node);
>   		b->first_wait = wait;
>   		smp_store_mb(b->tasklet, wait->tsk);
> -		first = __intel_breadcrumbs_enable_irq(b);
> +		/* After assigning ourselves as the new bottom-half, we must
> +		 * perform a cursory check to prevent a missed interrupt.
> +		 * Either we miss the interrupt whilst programming the hardware,
> +		 * or if there was a previous waiter (for a later seqno) they
> +		 * may be woken instead of us (due to the inherent race
> +		 * in the unlocked read of b->tasklet in the irq handler) and
> +		 * so we miss the wake up.
> +		 */
> +		__intel_breadcrumbs_enable_irq(b);
>   	}
>   	GEM_BUG_ON(!b->tasklet);
>   	GEM_BUG_ON(!b->first_wait);

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

Regards,

Tvrtko

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