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hrtimer: Preserve timer state in remove_hrtimer() (sqazi)

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 ca10949 (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>
Cc: stable@kernel.org
  • Loading branch information...
commit 42d3076cce709f50323aea4524b7cc1c5a6c7404 1 parent 1d365d3
@ch33kybutt ch33kybutt authored
Showing with 11 additions and 2 deletions.
  1. +11 −2 kernel/hrtimer.c
View
13 kernel/hrtimer.c
@@ -873,6 +873,7 @@ static inline int
remove_hrtimer(struct hrtimer *timer, struct hrtimer_clock_base *base)
{
if (hrtimer_is_queued(timer)) {
+ unsigned long state;
int reprogram;
/*
@@ -886,8 +887,13 @@ remove_hrtimer(struct hrtimer *timer, struct hrtimer_clock_base *base)
debug_hrtimer_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;
@@ -1173,6 +1179,9 @@ static void __run_hrtimer(struct hrtimer *timer)
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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