[PATCH] genirq: Warn about using IRQF_ONESHOT without a threaded handler

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

IRQF_ONESHOT disables the interrupt source until after the threaded
handler completed its work. This is needed to allow the threaded handler
to run - otherwise the CPU will get back to the interrupt handler
because the interrupt source remains active and the threaded handler
will not able to do its work.

Specifying IRQF_ONESHOT without a threaded handler does not make sense.
It could be a leftover if the handler _was_ threaded and changed back to
primary and the flag was not removed. This can be problematic in the
`threadirqs' case because the handler is exempt from forced-threading.
This in turn can become a problem on a PREEMPT_RT system if the handler
attempts to acquire sleeping locks.

Warn about missing threaded handlers with the IRQF_ONESHOT flag.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---

This popped up after Stefan Klug posted
   https://lore.kernel.org/all/20260105-sklug-v6-16-topic-dw100-v3-1-dev-v1-3-65af34d04fd8@ideasonboard.com/

There are a few drivers in tree which will trigger this warning such as
- arch/arm/mach-versatile/spc.c
- drivers/char/tpm/tpm_tis_spi_cr50.c
- drivers/edac/altera_edac.c
- drivers/i2c/busses/i2c-k1.c
- …

just to name a few. I *think* if IRQF_ONESHOT was on purpose and not
driven by copy/paste then the they wanted to use IRQF_NO_THREAD.

 kernel/irq/manage.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index 349ae7979da0e..18a8405cadb26 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -1473,6 +1473,13 @@ __setup_irq(unsigned int irq, struct irq_desc *desc, struct irqaction *new)
 	if (!(new->flags & IRQF_TRIGGER_MASK))
 		new->flags |= irqd_get_trigger_type(&desc->irq_data);
 
+	/*
+	 * IRQF_ONESHOT means the interrupt source in the IRQ chip will be
+	 * masked until the threaded handled is done. If there is no thread
+	 * handler then it makes no sense to have IRQF_ONESHOT.
+	 */
+	WARN_ON_ONCE(new->flags & IRQF_ONESHOT && !new->thread_fn);
+
 	/*
 	 * Check whether the interrupt nests into another interrupt
 	 * thread.
-- 
2.51.0