del_timer() vs. mod_timer() SMP race

Hi !

We just spent some days fighting a rare race in one of the distro's who backported some of timer.c from 2.6 to 2.4 (though they missed a bit).

The actual race we found didn't happen in 2.6 _but_ code inspection showed that a similar race is still present in 2.6, explanation below:

Code removing a timer from a list (run_timers or del_timer) takes that CPU list lock, does list_del, then timer->base = NULL.

It is mandatory that this timer->base = NULL is visible to other CPUs only after the list_del() is complete. If not, then mod timer could see it NULL, thus take it's own CPU list lock and not the one for the CPU the timer was beeing removed from the list, and thus the list_add in mod_timer() could race with the list_del() from run_timers() or del_timer().

Our race happened with run_timers(), which _DOES_ contain a proper smp_wmb() in the right spot in 2.6, but didn't in the "backport" we were fighting with.

However, del_timer() doesn't have such a barrier, and thus is subject to this race in 2.6 as well. This patch fixes it.

Index: linux-work/kernel/timer.c
===================================================================
@@ -308,6 +308,7 @@
 		goto repeat;
 	}
 	list_del(&timer->entry);
+	smp_wmb();
 	timer->base = NULL;
 	spin_unlock_irqrestore(&base->lock, flags);
 

Pretty please, always add a comment when putting an open-coded barrier into the kernel. Otherwise people cannot tell why it is there.

Ok, I also added the comment to the "other" smp_wmb() which was already there just in case...

It is mandatory that this timer->base = NULL is visible to other CPUs only after the list_del() is complete. If not, then mod timer could see it NULL, thus take it's own CPU list lock and not the one for the CPU the timer was beeing removed from the list, and thus the list_add in mod_timer() could race with the list_del() from run_timers() or del_timer().

Index: linux-work/kernel/timer.c
===================================================================
@@ -308,6 +308,10 @@
 		goto repeat;
 	}
 	list_del(&timer->entry);
+	smp_wmb(); /* the list del must have taken effect before timer->base
+		    * change is visible to other CPUs, or a concurrent mod_timer
+		    * would cause a race with list_add
+		    */
 	timer->base = NULL;
 	spin_unlock_irqrestore(&base->lock, flags);
 

@@ -460,7 +464,10 @@
 
 			list_del(&timer->entry);
 			set_running_timer(base, timer);
-			smp_wmb();
+			smp_wmb(); /* the list del must have taken effect before timer->base
+				    * change is visible to other CPUs, or a concurrent mod_timer
+				    * would cause a race with list_add
+				    */
 			timer->base = NULL;
 			spin_unlock_irq(&base->lock);
 			fn(data);