hrtimer: Preserve timer state in remove_hrtimer()

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>
Cc: stable@kernel.org

diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index 1decafb..72206cf 100644 (file)
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -931,6 +931,7 @@ static inline int
 remove_hrtimer(struct hrtimer *timer, struct hrtimer_clock_base *base)
 {
        if (hrtimer_is_queued(timer)) {
+               unsigned long state;
                int reprogram;
 
                /*
@@ -944,8 +945,13 @@ remove_hrtimer(struct hrtimer *timer, struct hrtimer_clock_base *base)
                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;
@@ -1231,6 +1237,9 @@ static void __run_hrtimer(struct hrtimer *timer, ktime_t *now)
                BUG_ON(timer->state != HRTIMER_STATE_CALLBACK);
                enqueue_hrtimer(timer, base);
        }
+
+       WARN_ON_ONCE(!(timer->state & HRTIMER_STATE_CALLBACK));
+
        timer->state &= ~HRTIMER_STATE_CALLBACK;
 }