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timer: Prevent base->clk from moving backward 

commit 30c66fc30ee7a98c4f3adf5fb7e213b61884474f upstream.

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: a683f390b9 ("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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Frederic Weisbecker 2020-07-03 03:06:57 +02:00 committed by Greg Kroah-Hartman
parent e9506de7b3
commit 6c2388e2a1
1 changed files with 14 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

17
kernel/time/timer.c
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@ -585,7 +585,15 @@ 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);
}
@ -897,10 +905,13 @@ 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
}