Quoting Sebastian Andrzej Siewior (2019-09-26 11:56:43)
The function intel_engine_breadcrumbs_irq() is always invoked from an interrupt handler and for that reason it invokes (as an optimisation) only spin_lock() for locking assuming that the interrupts are already disabled. The function intel_engine_signal_breadcrumbs() is provided to disable interrupts while the former function is invoked so that assumption is also true for callers from preemptible context.
On PREEMPT_RT local_irq_disable() really disables interrupts and this forbids to invoke spin_lock() which becomes a sleeping spinlock.
This is also problematic with `threadirqs' in conjunction with irq_work. With force threading the interrupt handler, the handler is invoked with disabled BH but with interrupts enabled. This is okay and the lock itself is never acquired in IRQ context. This changes with irq_work (signal_irq_work()) which _still_ invokes intel_engine_breadcrumbs_irq() from IRQ context. Lockdep should see this and complain.
Acquire the locks in intel_engine_breadcrumbs_irq() with _irqsave() suffix and let all callers invoke intel_engine_breadcrumbs_irq() directly instead using intel_engine_signal_breadcrumbs().
Reported-by: Clark Williams williams@redhat.com Signed-off-by: Sebastian Andrzej Siewior bigeasy@linutronix.de
All those irq save/restore look annoying, still the argument is valid Reviewed: Chris Wilson chris@chris-wilson.co.uk -Chris