Re: [PATCH 08/14] hrtimer: Allow hrtimer::function() to free the timer

[Oleg Nesterov]

On 06/05, Peter Zijlstra wrote:
>
>  #define HRTIMER_STATE_INACTIVE	0x00
>  #define HRTIMER_STATE_ENQUEUED	0x01
> -#define HRTIMER_STATE_CALLBACK	0x02
> -#define HRTIMER_STATE_MIGRATE	0x04
> +#define HRTIMER_STATE_MIGRATE	0x02

Slightly pff-topic, but I do not understand HRTIMER_STATE_MIGRATE
with or without this change. Unless I am totally confused it looks
buggy and simply unneeded.

migrate_hrtimer_list() sets it to keep hrtimer_active() == T, but
this is not enough: this can fool, say, hrtimer_is_queued() in
dequeue_signal().

Can't migrate_hrtimer_list() simply use HRTIMER_STATE_ENQUEUED?
This fixes the race and we can kill STATE_MIGRATE.

Oleg.


--- x/include/linux/hrtimer.h
+++ x/include/linux/hrtimer.h
@@ -70,17 +70,13 @@ enum hrtimer_restart {
  * the handling of the timer.
  *
  * The HRTIMER_STATE_ENQUEUED bit is always or'ed to the current state
- * to preserve the HRTIMER_STATE_CALLBACK in the above scenario. This
- * also affects HRTIMER_STATE_MIGRATE where the preservation is not
- * necessary. HRTIMER_STATE_MIGRATE is cleared after the timer is
- * enqueued on the new cpu.
+ * to preserve the HRTIMER_STATE_CALLBACK in the above scenario.
  *
  * All state transitions are protected by cpu_base->lock.
  */
 #define HRTIMER_STATE_INACTIVE	0x00
 #define HRTIMER_STATE_ENQUEUED	0x01
 #define HRTIMER_STATE_CALLBACK	0x02
-#define HRTIMER_STATE_MIGRATE	0x04
 
 /**
  * struct hrtimer - the basic hrtimer structure
--- x/kernel/time/hrtimer.c
+++ x/kernel/time/hrtimer.c
@@ -1642,11 +1642,11 @@ static void migrate_hrtimer_list(struct 
 		debug_deactivate(timer);
 
 		/*
-		 * Mark it as STATE_MIGRATE not INACTIVE otherwise the
+		 * Mark it as ENQUEUED not INACTIVE otherwise the
 		 * timer could be seen as !active and just vanish away
 		 * under us on another CPU
 		 */
-		__remove_hrtimer(timer, old_base, HRTIMER_STATE_MIGRATE, 0);
+		__remove_hrtimer(timer, old_base, HRTIMER_STATE_ENQUEUED, 0);
 		timer->base = new_base;
 		/*
 		 * Enqueue the timers on the new cpu. This does not
@@ -1657,9 +1657,6 @@ static void migrate_hrtimer_list(struct 
 		 * event device.
 		 */
 		enqueue_hrtimer(timer, new_base);
-
-		/* Clear the migration state bit */
-		timer->state &= ~HRTIMER_STATE_MIGRATE;
 	}
 }
 

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/