Re: [i915-rt v6 00/24] drm/i915/display: All patches to make PREEMPT_RT work on i915 + xe.

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2026-02-26 15:19:42 [+0100], To Maarten Lankhorst wrote:
> On 2026-02-26 13:07:18 [+0100], To Maarten Lankhorst wrote:
> > series somewhere I could pull and check. In meantime I would look what
> > causes the lockup on i915.
> 
> I think I got it.

This the atomic sync as-is, IRQ-Work (FIFO-1) will be preempted by the
threaded-interrupt (FIFO-50) and the interrupt will poll on
signaler_active while the irq-work can't make progress.

This will provide the needed sync:

diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
index a2b413982ce64..337f6e88faf05 100644
--- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
@@ -209,6 +209,7 @@ static void signal_irq_work(struct irq_work *work)
 		intel_breadcrumbs_disarm_irq(b);
 
 	rcu_read_lock();
+	spin_lock(&b->signaler_active_sync);
 	atomic_inc(&b->signaler_active);
 	list_for_each_entry_rcu(ce, &b->signalers, signal_link) {
 		struct i915_request *rq;
@@ -246,6 +247,7 @@ static void signal_irq_work(struct irq_work *work)
 		}
 	}
 	atomic_dec(&b->signaler_active);
+	spin_unlock(&b->signaler_active_sync);
 	rcu_read_unlock();
 
 	llist_for_each_safe(signal, sn, signal) {
@@ -290,6 +292,7 @@ intel_breadcrumbs_create(struct intel_engine_cs *irq_engine)
 	init_llist_head(&b->signaled_requests);
 
 	spin_lock_init(&b->irq_lock);
+	spin_lock_init(&b->signaler_active_sync);
 	init_irq_work(&b->irq_work, signal_irq_work);
 
 	b->irq_engine = irq_engine;
@@ -487,8 +490,11 @@ void intel_context_remove_breadcrumbs(struct intel_context *ce,
 	if (release)
 		intel_context_put(ce);
 
-	while (atomic_read(&b->signaler_active))
+	while (atomic_read(&b->signaler_active)) {
+		spin_lock(&b->signaler_active_sync);
+		spin_unlock(&b->signaler_active_sync);
 		cpu_relax();
+	}
 }
 
 static void print_signals(struct intel_breadcrumbs *b, struct drm_printer *p)
diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs_types.h b/drivers/gpu/drm/i915/gt/intel_breadcrumbs_types.h
index bdf09fd67b6e7..28dae32628aab 100644
--- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs_types.h
+++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs_types.h
@@ -40,6 +40,7 @@ struct intel_breadcrumbs {
 	struct list_head signalers;
 	struct llist_head signaled_requests;
 	atomic_t signaler_active;
+	spinlock_t signaler_active_sync;
 
 	spinlock_t irq_lock; /* protects the interrupt from hardirq context */
 	struct irq_work irq_work; /* for use from inside irq_lock */

Sebastian