[v2] drm/i915: Don't disable interrupts independently of the lock

Commit Message

Sebastian Andrzej Siewior Oct. 17, 2019, 8:40 a.m. UTC

The locks (active.lock and rq->lock) need to be taken with disabled
interrupts. This is done in i915_request_retire() by disabling the
interrupts independently of the locks itself.
While local_irq_disable()+spin_lock() equals spin_lock_irq() on vanilla
it does not on PREEMPT_RT.
Chris Wilson confirmed that local_irq_disable() was just introduced as
an optimisation to avoid enabling/disabling interrupts during
lock/unlock combo.

Enable/disable interrupts as part of the locking instruction.

Cc: Chris Wilson <[e-mail]>
Signed-off-by: Sebastian Andrzej Siewior <[e-mail]>
---
v1…v2: Rebase to v5.4-rc3

 drivers/gpu/drm/i915/i915_request.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

Comments

Chris Wilson Oct. 17, 2019, 8:42 a.m. UTC | #1

Quoting Sebastian Andrzej Siewior (2019-10-17 09:40:01)
> The locks (active.lock and rq->lock) need to be taken with disabled
> interrupts. This is done in i915_request_retire() by disabling the
> interrupts independently of the locks itself.
> While local_irq_disable()+spin_lock() equals spin_lock_irq() on vanilla
> it does not on PREEMPT_RT.
> Chris Wilson confirmed that local_irq_disable() was just introduced as
> an optimisation to avoid enabling/disabling interrupts during
> lock/unlock combo.
> 
> Enable/disable interrupts as part of the locking instruction.
> 
> Cc: Chris Wilson <[e-mail]>
> Signed-off-by: Sebastian Andrzej Siewior <[e-mail]>
Reviewed-by: Chris Wilson <[e-mail]>
-Chris

Patch

diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index 1c5506822dc72..cf10246fb01ad 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -205,14 +205,14 @@  static void remove_from_engine(struct i915_request *rq)
 	 * check that the rq still belongs to the newly locked engine.
 	 */
 	locked = READ_ONCE(rq->engine);
-	spin_lock(&locked->active.lock);
+	spin_lock_irq(&locked->active.lock);
 	while (unlikely(locked != (engine = READ_ONCE(rq->engine)))) {
 		spin_unlock(&locked->active.lock);
 		spin_lock(&engine->active.lock);
 		locked = engine;
 	}
 	list_del(&rq->sched.link);
-	spin_unlock(&locked->active.lock);
+	spin_unlock_irq(&locked->active.lock);
 }
 
 static bool i915_request_retire(struct i915_request *rq)
@@ -272,8 +272,6 @@  static bool i915_request_retire(struct i915_request *rq)
 		active->retire(active, rq);
 	}
 
-	local_irq_disable();
-
 	/*
 	 * We only loosely track inflight requests across preemption,
 	 * and so we may find ourselves attempting to retire a _completed_
@@ -282,7 +280,7 @@  static bool i915_request_retire(struct i915_request *rq)
 	 */
 	remove_from_engine(rq);
 
-	spin_lock(&rq->lock);
+	spin_lock_irq(&rq->lock);
 	i915_request_mark_complete(rq);
 	if (!i915_request_signaled(rq))
 		dma_fence_signal_locked(&rq->fence);
@@ -297,9 +295,7 @@  static bool i915_request_retire(struct i915_request *rq)
 		__notify_execute_cb(rq);
 	}
 	GEM_BUG_ON(!list_empty(&rq->execute_cb));
-	spin_unlock(&rq->lock);
-
-	local_irq_enable();
+	spin_unlock_irq(&rq->lock);
 
 	remove_from_client(rq);
 	list_del(&rq->link);