[PATCH] timer: Prevent base->clk from moving backward

[PATCH] timer: Prevent base->clk from moving backward

[Frederic Weisbecker]

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.

Fixes: a683f390b93f ("timers: Forward the wheel clock whenever possible")
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Juri Lelli <juri.lelli@redhat.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

Re: [PATCH] timer: Prevent base->clk from moving backward

[Anna-Maria Behnsen]

Hi,

On Fri, 3 Jul 2020, Frederic Weisbecker wrote:

> 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.
> 
> Fixes: a683f390b93f ("timers: Forward the wheel clock whenever possible")
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> ---

Reviewed-by: Anna-Maria Behnsen <anna-maria@linutronix.de>

Thanks,

	Anna-Maria

[tip: timers/urgent] timer: Prevent base->clk from moving backward

[tip-bot2 for Frederic Weisbecker]

The following commit has been merged into the timers/urgent branch of tip:

Commit-ID:     30c66fc30ee7a98c4f3adf5fb7e213b61884474f
Gitweb:        https://git.kernel.org/tip/30c66fc30ee7a98c4f3adf5fb7e213b61884474f
Author:        Frederic Weisbecker <frederic@kernel.org>
AuthorDate:    Fri, 03 Jul 2020 03:06:57 +02:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Thu, 09 Jul 2020 11:56:57 +02:00

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.

Fixes: a683f390b93f ("timers: Forward the wheel clock whenever possible")
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Tested-by: Juri Lelli <juri.lelli@redhat.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20200703010657.2302-1-frederic@kernel.org
---
 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 398e6ea..9a838d3 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
 }