[Intel-gfx] [PATCH] drm/i915: Wake up the bottom-half if we steal their interrupt

[Chris Wilson]

On Tue, Jul 05, 2016 at 12:45:55PM +0100, Chris Wilson wrote:
> Following on from the scenario Tvrtko envision to explain a hard-to-hit
> race with multiple first waiters, we could also then race in the
> __i915_request_irq_complete() and the bottom-half may miss the vital
> irq-seqno barrier and so go to sleep not noticing their seqno is
> complete.
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> ---
>  drivers/gpu/drm/i915/i915_drv.h | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index c269e0a..9a5498c 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -3999,6 +3999,8 @@ static inline bool __i915_request_irq_complete(struct drm_i915_gem_request *req)
>  	 */
>  	if (engine->irq_seqno_barrier &&
>  	    cmpxchg_relaxed(&engine->irq_posted, 1, 0)) {
> +		struct task_struct *tsk;
> +
>  		/* The ordering of irq_posted versus applying the barrier
>  		 * is crucial. The clearing of the current irq_posted must
>  		 * be visible before we perform the barrier operation,
> @@ -4012,6 +4014,24 @@ static inline bool __i915_request_irq_complete(struct drm_i915_gem_request *req)
>  		 * seqno update.
>  		 */
>  		engine->irq_seqno_barrier(engine);
> +
> +		/* If we consume the irq, but we are no longer the bottom-half,
> +		 * the real bottom-half may not have serialised their own
> +		 * seqno check with the irq-barrier (i.e. may have inspected
> +		 * the seqno before we believe it coherent).
> +		 */
> +		rcu_read_lock();
> +		tsk = READ_ONCE(engine->breadcrumbs.tasklet);
> +		if (tsk != current)
> +			/* Note that if the bottom-half is changed as we
> +			 * are sending the wake-up, the new bottom-half will
> +			 * be woken by whomever made the change. We only have
> +			 * to worry about when we steal the irq-posted for
> +			 * ourself.
> +			 */
> +			wake_up_process(tsk);
> +		rcu_read_lock();

Please pretend that I wrote rcu_read_unlock(). Thankyou.
-Chris
>  

-- 
Chris Wilson, Intel Open Source Technology Centre