[PATCH] genirq: Fix race on spurious interrupt detection

[Lukas Wunner]

Commit 1e77d0a1ed74 ("genirq: Sanitize spurious interrupt detection of
threaded irqs") made detection of spurious interrupts work for threaded
handlers by:

a) incrementing a counter every time the thread returns IRQ_HANDLED, and
b) checking whether that counter has increased every time the thread is
   woken.

However for oneshot interrupts, the commit unmasks the interrupt before
incrementing the counter.  If another interrupt occurs right after
unmasking but before the counter is incremented, that interrupt is
incorrectly considered spurious:

time
 |  irq_thread()
 |    irq_thread_fn()
 |      action->thread_fn()
 |      irq_finalize_oneshot()
 |        unmask_threaded_irq()            /* interrupt is unmasked */
 |
 |                  /* interrupt fires, incorrectly deemed spurious */
 |
 |    atomic_inc(&desc->threads_handled); /* counter is incremented */
 v

I am seeing this with a hi3110 CAN controller receiving data at high
volume (from a separate machine sending with "cangen -g 0 -i -x"):
The controller signals a huge number of interrupts (hundreds of millions
per day) and every second there are about a dozen which are deemed
spurious.  The issue is benign in this case, mostly just an irritation,
but I'm worrying that at high CPU load and in the presence of higher
priority tasks, the number of incorrectly detected spurious interrupts
might increase beyond the 99,900 threshold and cause disablement of the
IRQ.

Fix by moving the incrementation of the counter from irq_thread() to
irq_thread_fn() and irq_forced_thread_fn(), before the invocation of
irq_finalize_oneshot().

Fixes: 1e77d0a1ed74 ("genirq: Sanitize spurious interrupt detection of threaded irqs")
Signed-off-by: Lukas Wunner <lukas@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # v3.16+
---
 kernel/irq/manage.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index fb86146037a7..9dbdccab3b6a 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -927,6 +927,9 @@ irq_forced_thread_fn(struct irq_desc *desc, struct irqaction *action)
 
 	local_bh_disable();
 	ret = action->thread_fn(action->irq, action->dev_id);
+	if (ret == IRQ_HANDLED)
+		atomic_inc(&desc->threads_handled);
+
 	irq_finalize_oneshot(desc, action);
 	local_bh_enable();
 	return ret;
@@ -943,6 +946,9 @@ static irqreturn_t irq_thread_fn(struct irq_desc *desc,
 	irqreturn_t ret;
 
 	ret = action->thread_fn(action->irq, action->dev_id);
+	if (ret == IRQ_HANDLED)
+		atomic_inc(&desc->threads_handled);
+
 	irq_finalize_oneshot(desc, action);
 	return ret;
 }
@@ -1020,8 +1026,6 @@ static int irq_thread(void *data)
 		irq_thread_check_affinity(desc, action);
 
 		action_ret = handler_fn(desc, action);
-		if (action_ret == IRQ_HANDLED)
-			atomic_inc(&desc->threads_handled);
 		if (action_ret == IRQ_WAKE_THREAD)
 			irq_wake_secondary(desc, action);
 
-- 
2.19.1