Messages in this thread Patch in this message | | | Subject | [PATCH] Fix a complex race in hrtimer code. | From | Salman Qazi <> | Date | Thu, 07 Oct 2010 19:33:56 -0700 |
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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
Bug reproducible and fix testable using a kernel module that hrtimer_start()s a timer with a very long running callback from multiple CPUs:
MODULE_LICENSE("GPL");
static long timer_func_time = 1000; module_param(timer_func_time, long, 0600); static long timer_starts = 2500; module_param(timer_starts, long, 0600); static long timer_expire = 1000; module_param(timer_expire, long, 0600);
DEFINE_PER_CPU(struct task_struct *, hrtimer_thread);
/* There are other issues, like deadlocks between multiple hrtimer_start observed * calls, at least in 2.6.34 that this lock works around. Will look into * those later. */ static DEFINE_SPINLOCK(blah_lock);
static ktime_t mytime; static struct hrtimer timer;
static enum hrtimer_restart timer_restart(struct hrtimer *timer) { unsigned long i; /* We have to make the callback longer to improve the * probability of having a race. */ for (i = 0; i < timer_func_time / 100; i++) { touch_nmi_watchdog(); touch_softlockup_watchdog(); udelay(100); }
return HRTIMER_NORESTART; }
static int restart_hrtimer_repeatedly(void *input) { int i; unsigned long range; while (!kthread_should_stop()) { for (i = 0; i < timer_starts; i++) { /* Avoid deadlocks for now */ spin_lock(&blah_lock); hrtimer_start(&timer, mytime, HRTIMER_MODE_REL); spin_unlock(&blah_lock); touch_nmi_watchdog(); touch_softlockup_watchdog(); } cond_resched(); } hrtimer_cancel(&timer); return 0; }
static int hrtimer_unit_test_init(void) { int cpu; mytime = ktime_set(0, 0); mytime = ktime_add_ns(mytime, timer_expire); hrtimer_init(&timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL); timer.function = timer_restart; for_each_online_cpu(cpu) { per_cpu(hrtimer_thread, cpu) = kthread_create( restart_hrtimer_repeatedly, NULL, "hrtimer_test/%d", cpu); if (IS_ERR(per_cpu(hrtimer_thread, cpu))) { printk(KERN_ERR "Failed to create hrtimer test thread\n"); BUG(); } kthread_bind(per_cpu(hrtimer_thread, cpu), cpu); wake_up_process(per_cpu(hrtimer_thread, cpu)); } return 0; }
static void hrtimer_unit_test_exit(void) { int cpu; for_each_online_cpu(cpu) { (void)kthread_stop(per_cpu(hrtimer_thread, cpu)); } }
module_init(hrtimer_unit_test_init); module_exit(hrtimer_unit_test_exit); --- kernel/hrtimer.c | 7 +++++-- 1 files changed, 5 insertions(+), 2 deletions(-)
diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c index 1decafb..4769c51 100644 --- a/kernel/hrtimer.c +++ b/kernel/hrtimer.c @@ -944,8 +944,8 @@ 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); + __remove_hrtimer(timer, base, + (timer->state & HRTIMER_STATE_CALLBACK), reprogram); return 1; } return 0; @@ -1231,6 +1231,9 @@ static void __run_hrtimer(struct hrtimer *timer, ktime_t *now) BUG_ON(timer->state != HRTIMER_STATE_CALLBACK); enqueue_hrtimer(timer, base); } + + BUG_ON(!(timer->state & HRTIMER_STATE_CALLBACK)); + timer->state &= ~HRTIMER_STATE_CALLBACK; }
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