[RFC 1/2] watchdog: Check WDOG_STOP_ON_REBOOT in reboot notifier

[Dmitry Safonov <dima@arista.com>]

Many watchdog drivers use watchdog_stop_on_reboot() helper in order
to stop the watchdog on system reboot. Unfortunately, this logic is
coded in driver's probe function and doesn't allows user to decide what
to do during shutdown/reboot.

On the other side, Xen and Qemu watchdog drivers (xen_wdt and i6300esb)
may be configured to either send NMI or turn off/reboot VM as
the watchdog action. As the kernel may stuck at any state, sending NMIs
can't reliably reboot the VM.

At Arista, we benefited from the following set-up: the emulated watchdogs
trigger VM reset and softdog is set to catch less severe conditions to
generate vmcore. Just before reboot watchdog's timeout is increased
to some good-enough value (3 mins). That keeps watchdog always running
and guarantees that VM doesn't stuck.

As a preparation to move the watchdog's decision to stop on reboot or
not in userspace, allow WDOG_STOP_ON_REBOOT to be set during runtime,
not only on driver's probing. Always register reboot notifier and check
WDOG_STOP_ON_REBOOT inside it (on actual reboot).

Signed-off-by: Dmitry Safonov <dima@arista.com>
---
 drivers/watchdog/watchdog_dev.c | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
index 4b2a85438478..8766dd93028f 100644
--- a/drivers/watchdog/watchdog_dev.c
+++ b/drivers/watchdog/watchdog_dev.c
@@ -1103,6 +1103,10 @@ static int watchdog_reboot_notifier(struct notifier_block *nb,
 	struct watchdog_device *wdd;
 
 	wdd = container_of(nb, struct watchdog_device, reboot_nb);
+
+	if (!test_bit(WDOG_STOP_ON_REBOOT, &wdd->status))
+		return NOTIFY_DONE;
+
 	if (code == SYS_DOWN || code == SYS_HALT) {
 		if (watchdog_active(wdd)) {
 			int ret;
@@ -1139,16 +1143,13 @@ int watchdog_dev_register(struct watchdog_device *wdd)
 		return ret;
 	}
 
-	if (test_bit(WDOG_STOP_ON_REBOOT, &wdd->status)) {
-		wdd->reboot_nb.notifier_call = watchdog_reboot_notifier;
+	wdd->reboot_nb.notifier_call = watchdog_reboot_notifier;
 
-		ret = devm_register_reboot_notifier(&wdd->wd_data->dev,
-						    &wdd->reboot_nb);
-		if (ret) {
-			pr_err("watchdog%d: Cannot register reboot notifier (%d)\n",
-			       wdd->id, ret);
-			watchdog_dev_unregister(wdd);
-		}
+	ret = devm_register_reboot_notifier(&wdd->wd_data->dev, &wdd->reboot_nb);
+	if (ret) {
+		pr_err("watchdog%d: Cannot register reboot notifier (%d)\n",
+				wdd->id, ret);
+		watchdog_dev_unregister(wdd);
 	}
 
 	return ret;
-- 
2.25.0