On 2026-03-10 12:57:08 [+0100], Maarten Lankhorst wrote: > From: Sebastian Andrzej Siewior <[email protected]> > > 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.
The threaded-interrupt is the interrupt. | On PREEMPT_RT the irq_work can be preempted by threaded-interrupt which | will be poll for completion but the irq_work routine can't make | progress. > Solve this by adding a spinlock to prevent starvation and force > completion. | Solve this by adding a spinlock_t to prevent starvation by forcing a | context switch if lock is held based on `signaler_active'. On | !PREEMPT_RT `signaler_active' can only be non-zero if multiple CPUs are | involved and spinning on the lock leds to the same result. > Signed-off-by: Maarten Lankhorst <[email protected]> If I am the From: then I should have the Signed-off-by, too. Let me do Signed-off-by: Sebastian Andrzej Siewior <[email protected]> so it can be picked up. You did suggest the following: --- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c +++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c @@ -209,7 +209,7 @@ static void signal_irq_work(struct irq_work *work) intel_breadcrumbs_disarm_irq(b); rcu_read_lock(); - atomic_inc(&b->signaler_active); + spin_lock(&b->signaler_active_sync); list_for_each_entry_rcu(ce, &b->signalers, signal_link) { struct i915_request *rq; @@ -245,7 +245,7 @@ static void signal_irq_work(struct irq_work *work) i915_request_put(rq); } } - atomic_dec(&b->signaler_active); + spin_unlock(&b->signaler_active_sync); rcu_read_unlock(); llist_for_each_safe(signal, sn, signal) { @@ -290,6 +290,7 @@ 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 +488,10 @@ void intel_context_remove_breadcrumbs(struct intel_context *ce, if (release) intel_context_put(ce); - while (atomic_read(&b->signaler_active)) - cpu_relax(); + while (spin_is_locked(&b->signaler_active_sync)) { + spin_lock_irqsave(&b->signaler_active_sync, flags); + spin_unlock_irqrestore(&b->signaler_active_sync, flags); And this does not work because spin_is_locked() returns true and spins forever. This fails because there is a "corner case" where spin_is_locked() returns but the lock has no lock owner as in locked. This happens if there is a waiter which did not yet acquire the lock. So if you happy with this, we could keep it ;) + } } 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..d071f4f1b1910 100644 --- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs_types.h +++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs_types.h @@ -39,7 +39,7 @@ struct intel_breadcrumbs { spinlock_t signalers_lock; /* protects the list of signalers */ 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
