[PATCH] gpio: virtuser: skip free_irq when no IRQ is installed

Disabling interrupt monitoring uses atomic_xchg() to clear the stored IRQ.
When monitoring is already disabled, atomic_xchg() returns 0. It must not
be passed to free_irq().

The bug is reproducible on an x86_64 QEMU guest with
CONFIG_GPIO_VIRTUSER=y and CONFIG_GPIO_SIM=y. Configure a live
gpio-virtuser device through configfs. Its input lookup must refer to a
live gpio-sim bank, such as key gpio-sim-test with offset 0. The
consumer's dev_name attribute is shown as <dev> below; then run:

    echo 0 > /sys/kernel/debug/gpio-virtuser/<dev>/gpiod:input:0/interrupts

On an unpatched kernel, this reaches gpio_virtuser_interrupts_set() with
ld->irq still at its initial value 0, and free_irq() reports:

    Trying to free already-free IRQ 0

The same reproducer completes without the warning on the patched kernel.

Fixes: 91581c4b3f29 ("gpio: virtuser: new virtual testing driver for the GPIO API")
Assisted-by: LLM Codex
Signed-off-by: Runyu Xiao <redacted>
---
 drivers/gpio/gpio-virtuser.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpio/gpio-virtuser.c b/drivers/gpio/gpio-virtuser.c
index 7d0d366be..d5876c19c 100644
--- a/drivers/gpio/gpio-virtuser.c
+++ b/drivers/gpio/gpio-virtuser.c

@@ -692,7 +692,8 @@ static int gpio_virtuser_interrupts_s et(void *data, u64 val)
 		atomic_set(&ld->irq, irq);
 	} else {
 		irq = atomic_xchg(&ld->irq, 0);
-		free_irq(irq, ld);
+		if (irq)
+			free_irq(irq, ld);
 	}
 
 	return 0;

-- 
2.34.1