linux-kernel - [RFC PATCH 01/10] timer: Prevent base->clk from moving backward

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Message-Id: <20200701011030.14324-2-frederic@kernel.org>
Date:   Wed,  1 Jul 2020 03:10:21 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: [RFC PATCH 01/10] timer: Prevent base->clk from moving backward

When a timer is enqueued with a negative delta (ie: expiry is below
base->clk), it gets added to the wheel as expiring now (base->clk).

Yet the value that gets stored in base->next_expiry, while calling
trigger_dyntick_cpu(), is the initial timer->expires value. The
resulting state becomes:

	base->next_expiry < base->clk

On the next timer enqueue, forward_timer_base() may accidentally
rewind base->clk. As a possible outcome, timers may expire way too
early, the worst case being that the highest wheel levels get spuriously
processed again.

To prevent from that, make sure that base->next_expiry doesn't get below
base->clk.

Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Anna-Maria Gleixner <anna-maria@...utronix.de>
Cc: Juri Lelli <juri.lelli@...hat.com>
---
 kernel/time/timer.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 398e6eadb861..9a838d38dbe6 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -584,7 +584,15 @@ trigger_dyntick_cpu(struct timer_base *base, struct timer_list *timer)
 	 * Set the next expiry time and kick the CPU so it can reevaluate the
 	 * wheel:
 	 */
-	base->next_expiry = timer->expires;
+	if (time_before(timer->expires, base->clk)) {
+		/*
+		 * Prevent from forward_timer_base() moving the base->clk
+		 * backward
+		 */
+		base->next_expiry = base->clk;
+	} else {
+		base->next_expiry = timer->expires;
+	}
 	wake_up_nohz_cpu(base->cpu);
 }
 
@@ -896,10 +904,13 @@ static inline void forward_timer_base(struct timer_base *base)
 	 * If the next expiry value is > jiffies, then we fast forward to
 	 * jiffies otherwise we forward to the next expiry value.
 	 */
-	if (time_after(base->next_expiry, jnow))
+	if (time_after(base->next_expiry, jnow)) {
 		base->clk = jnow;
-	else
+	} else {
+		if (WARN_ON_ONCE(time_before(base->next_expiry, base->clk)))
+			return;
 		base->clk = base->next_expiry;
+	}
 #endif
 }
 
-- 
2.26.2