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Commit 99fe4a5f authored by Chris Wilson's avatar Chris Wilson
Browse files

drm/i915: Wake up the bottom-half if we steal their interrupt 



Following on from the scenario Tvrtko envisioned 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.

v2: unlock, not double lock the rcu_read_lock.

Fixes: 3d5564e9 ("drm/i915: Only apply one barrier after...")
Signed-off-by: default avatarChris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: default avatarTvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1467805142-22219-2-git-send-email-chris@chris-wilson.co.uk
parent 04171313

drivers/gpu/drm/i915/i915_drv.h

+22 −0
Original line number Diff line number Diff line
@@ -3998,7 +3998,10 @@ static inline bool __i915_request_irq_complete(struct drm_i915_gem_request *req) 
	 * is woken.

	 */

	if (engine->irq_seqno_barrier &&

	    READ_ONCE(engine->breadcrumbs.tasklet) == current &&

	    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 +4015,25 @@ 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 since they see

		 * irq_posted == false but we are still running).

		 */

		rcu_read_lock();

		tsk = READ_ONCE(engine->breadcrumbs.tasklet);

		if (tsk && 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_unlock();



		if (i915_gem_request_completed(req))

			return true;

	}