[PATCH] watchdog: wdt_pci: Fix shared IRQ storm and complete system lockup

[w15303746062]

From: Mingyu Wang <25181214217@xxxxxxxxxxxxxxxxx>

The wdt_pci driver registers its interrupt handler with the IRQF_SHARED
flag. However, the interrupt handler wdtpci_interrupt() fails to check
whether the interrupt actually originated from the watchdog device.

If another device on the same shared IRQ line (e.g., an I2C controller)
triggers an interrupt, wdtpci_interrupt() will erroneously process it,
blindly log hardware status (e.g., "Reset in 5ms") to the console, and
unconditionally return IRQ_HANDLED.

This behavior defeats the kernel's spurious interrupt detector. Under
heavy load from other devices sharing the IRQ, it causes a severe printk
storm and keeps the CPU trapped in hard IRQ context. This eventually
leads to a complete system lockup and RCU/Hung Task panics.

Fix this by checking the WDC_SR_IRQ bit (which is active low) in the
status register. If the bit is high, the interrupt is not ours, and we
must release the lock and return IRQ_NONE immediately.

Signed-off-by: Mingyu Wang <25181214217@xxxxxxxxxxxxxxxxx>
---
 drivers/watchdog/wdt_pci.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/watchdog/wdt_pci.c b/drivers/watchdog/wdt_pci.c
index 3918a600f2a0..a35ac064b690 100644
--- a/drivers/watchdog/wdt_pci.c
+++ b/drivers/watchdog/wdt_pci.c
@@ -304,6 +304,17 @@ static irqreturn_t wdtpci_interrupt(int irq, void *dev_id)
 	spin_lock(&wdtpci_lock);
 
 	status = inb(WDT_SR);
+	/*
+	 * The WDT500/501 supports shared interrupts (IRQF_SHARED).
+	 * We must check if the interrupt was generated by this device.
+	 * WDC_SR_IRQ is active low, so if it is set (1), the interrupt
+	 * belongs to another device on the shared line.
+	 */
+	if (status & WDC_SR_IRQ) {
+		spin_unlock(&wdtpci_lock);
+		return IRQ_NONE;
+	}
+
 	udelay(8);
 
 	pr_crit("status %d\n", status);
-- 
2.34.1