From da11a4140d575b4486113ea80a344c2f10526634 Mon Sep 17 00:00:00 2001
Message-Id: <da11a4140d575b4486113ea80a344c2f10526634.1592846147.git.zanussi@kernel.org>
In-Reply-To: <07cd0dbc80b976663c80755496a03f288decfe5a.1592846146.git.zanussi@kernel.org>
References: <07cd0dbc80b976663c80755496a03f288decfe5a.1592846146.git.zanussi@kernel.org>
From: Julia Cartwright <julia@ni.com>
Date: Fri, 28 Sep 2018 21:03:51 +0000
Subject: [PATCH 251/330] watchdog: prevent deferral of watchdogd wakeup on RT
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/4.19/older/patches-4.19.127-rt55.tar.xz

When PREEMPT_RT_FULL is enabled, all hrtimer expiry functions are
deferred for execution into the context of ktimersoftd unless otherwise
annotated.

Deferring the expiry of the hrtimer used by the watchdog core, however,
is a waste, as the callback does nothing but queue a kthread work item
and wakeup watchdogd.

It's worst then that, too: the deferral through ktimersoftd also means
that for correct behavior a user must adjust the scheduling parameters
of both watchdogd _and_ ktimersoftd, which is unnecessary and has other
side effects (like causing unrelated expiry functions to execute at
potentially elevated priority).

Instead, mark the hrtimer used by the watchdog core as being _HARD to
allow it's execution directly from hardirq context.  The work done in
this expiry function is well-bounded and minimal.

A user still must adjust the scheduling parameters of the watchdogd
to be correct w.r.t. their application needs.

Cc: Guenter Roeck <linux@roeck-us.net>
Reported-and-tested-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Reported-by: Tim Sander <tim@krieglstein.org>
Signed-off-by: Julia Cartwright <julia@ni.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
[bigeasy: use only HRTIMER_MODE_REL_HARD]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 drivers/watchdog/watchdog_dev.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
index 10b2090f3e5e..c04384aa94a5 100644
--- a/drivers/watchdog/watchdog_dev.c
+++ b/drivers/watchdog/watchdog_dev.c
@@ -145,7 +145,7 @@ static inline void watchdog_update_worker(struct watchdog_device *wdd)
 		ktime_t t = watchdog_next_keepalive(wdd);
 
 		if (t > 0)
-			hrtimer_start(&wd_data->timer, t, HRTIMER_MODE_REL);
+			hrtimer_start(&wd_data->timer, t, HRTIMER_MODE_REL_HARD);
 	} else {
 		hrtimer_cancel(&wd_data->timer);
 	}
@@ -164,7 +164,7 @@ static int __watchdog_ping(struct watchdog_device *wdd)
 	if (ktime_after(earliest_keepalive, now)) {
 		hrtimer_start(&wd_data->timer,
 			      ktime_sub(earliest_keepalive, now),
-			      HRTIMER_MODE_REL);
+			      HRTIMER_MODE_REL_HARD);
 		return 0;
 	}
 
@@ -948,7 +948,7 @@ static int watchdog_cdev_register(struct watchdog_device *wdd)
 		return -ENODEV;
 
 	kthread_init_work(&wd_data->work, watchdog_ping_work);
-	hrtimer_init(&wd_data->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+	hrtimer_init(&wd_data->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL_HARD);
 	wd_data->timer.function = watchdog_timer_expired;
 
 	if (wdd->id == 0) {
@@ -1005,7 +1005,7 @@ static int watchdog_cdev_register(struct watchdog_device *wdd)
 		__module_get(wdd->ops->owner);
 		get_device(&wd_data->dev);
 		if (handle_boot_enabled)
-			hrtimer_start(&wd_data->timer, 0, HRTIMER_MODE_REL);
+			hrtimer_start(&wd_data->timer, 0, HRTIMER_MODE_REL_HARD);
 		else
 			pr_info("watchdog%d running and kernel based pre-userspace handler disabled\n",
 				wdd->id);
-- 
2.17.1

