Commit 30c66fc3 authored Jul 03, 2020 by Frederic Weisbecker Committed by Thomas Gleixner Jul 09, 2020

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: a683f390 ("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

parent dcb7fd82

kernel/time/timer.c

+14 −3

Original line number	Diff line number	Diff line
		@@ -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:
		*/
		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
		}

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