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Subject[27/66] hrtimer: Preserve timer state in remove_hrtimer()
2.6.32-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Salman Qazi <sqazi@google.com>

commit f13d4f979c518119bba5439dd2364d76d31dcd3f upstream.

The race is described as follows:

CPU X CPU Y
remove_hrtimer
// state & QUEUED == 0
timer->state = CALLBACK
unlock timer base
timer->f(n) //very long
hrtimer_start
lock timer base
remove_hrtimer // no effect
hrtimer_enqueue
timer->state = CALLBACK |
QUEUED
unlock timer base
hrtimer_start
lock timer base
remove_hrtimer
mode = INACTIVE
// CALLBACK bit lost!
switch_hrtimer_base
CALLBACK bit not set:
timer->base
changes to a
different CPU.
lock this CPU's timer base
The bug was introduced with commit ca109491f (hrtimer: removing all ur
callback modes) in 2.6.29

[ tglx: Feed new state via local variable and add a comment. ]

Signed-off-by: Salman Qazi <sqazi@google.com>
Cc: akpm@linux-foundation.org
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <20101012142351.8485.21823.stgit@dungbeetle.mtv.corp.google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
kernel/hrtimer.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -920,6 +920,7 @@ static inline int
remove_hrtimer(struct hrtimer *timer, struct hrtimer_clock_base *base)
{
if (hrtimer_is_queued(timer)) {
+ unsigned long state;
int reprogram;

/*
@@ -933,8 +934,13 @@ remove_hrtimer(struct hrtimer *timer, st
debug_deactivate(timer);
timer_stats_hrtimer_clear_start_info(timer);
reprogram = base->cpu_base == &__get_cpu_var(hrtimer_bases);
- __remove_hrtimer(timer, base, HRTIMER_STATE_INACTIVE,
- reprogram);
+ /*
+ * We must preserve the CALLBACK state flag here,
+ * otherwise we could move the timer base in
+ * switch_hrtimer_base.
+ */
+ state = timer->state & HRTIMER_STATE_CALLBACK;
+ __remove_hrtimer(timer, base, state, reprogram);
return 1;
}
return 0;
@@ -1221,6 +1227,9 @@ static void __run_hrtimer(struct hrtimer
BUG_ON(timer->state != HRTIMER_STATE_CALLBACK);
enqueue_hrtimer(timer, base);
}
+
+ WARN_ON_ONCE(!(timer->state & HRTIMER_STATE_CALLBACK));
+
timer->state &= ~HRTIMER_STATE_CALLBACK;
}




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