From: Frederic Weisbecker <frederic@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
Frederic Weisbecker <frederic@kernel.org>,
Anna-Maria Gleixner <anna-maria@linutronix.de>,
Peter Zijlstra <peterz@infradead.org>,
Juri Lelli <juri.lelli@redhat.com>, Stable <stable@kernel.org>
Subject: [PATCH] timer: Prevent base->clk from moving backward
Date: Fri, 3 Jul 2020 03:06:57 +0200 [thread overview]
Message-ID: <20200703010657.2302-1-frederic@kernel.org> (raw)
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
next reply other threads:[~2020-07-03 1:07 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-03 1:06 Frederic Weisbecker [this message]
2020-07-09 9:42 ` [PATCH] timer: Prevent base->clk from moving backward Anna-Maria Behnsen
2020-07-09 10:04 ` [tip: timers/urgent] " tip-bot2 for Frederic Weisbecker
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