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Hey Ingo,
So we've been seeing the following trace fairly frequently on our SMP
boxes when running kernbench:

BUG: at kernel/softirq.c:639 __tasklet_action()

Call Trace:
[<ffffffff8106d5da>] dump_trace+0xaa/0x32a
[<ffffffff8106d89b>] show_trace+0x41/0x5c
[<ffffffff8106d8cb>] dump_stack+0x15/0x17
[<ffffffff81094a97>] __tasklet_action+0xdf/0x12e
[<ffffffff81094f76>] tasklet_action+0x27/0x29
[<ffffffff8109530a>] ksoftirqd+0x16c/0x271
[<ffffffff81033d4d>] kthread+0xf5/0x128
[<ffffffff8105ff68>] child_rip+0xa/0x12

Paul also pointed this out awhile back:

Anyway, I think I finally found the issue. Its a bit hard to explain,
but the idea is while __tasklet_action is running the tasklet function
on CPU1, if a call to tasklet_schedule() on CPU2 is made, and if right
after we mark the TASKLET_STATE_SCHED bit we are preempted,
__tasklet_action on CPU1 might be able to re-run the function, clear the
bit and unlock the tasklet before CPU2 enters __tasklet_common_schedule.
Once __tasklet_common_schedule locks the tasklet, we will add the
tasklet to the list with the TASKLET_STATE_SCHED *unset*.

I've verified this race occurs w/ a WARN_ON in

This fix avoids this race by making sure *after* we've locked the
tasklet that the STATE_SCHED bit is set before adding it to the list.

Does it look ok to you?


Signed-off-by: John Stultz <>

Index: 2.6-rt/kernel/softirq.c
--- 2.6-rt.orig/kernel/softirq.c 2007-06-05 18:30:54.000000000 -0700
+++ 2.6-rt/kernel/softirq.c 2007-06-05 18:36:44.000000000 -0700
@@ -544,10 +544,17 @@ static void inline
__tasklet_common_schedule(struct tasklet_struct *t, struct tasklet_head *head, unsigned int nr)
if (tasklet_trylock(t)) {
- WARN_ON(t->next != NULL);
- t->next = head->list;
- head->list = t;
- raise_softirq_irqoff(nr);
+ /* We may have been preempted before tasklet_trylock
+ * and __tasklet_action may have already run.
+ * So double check the sched bit while the takslet
+ * is locked before adding it to the list.
+ */
+ if (test_bit(TASKLET_STATE_SCHED, &t->state)) {
+ WARN_ON(t->next != NULL);
+ t->next = head->list;
+ head->list = t;
+ raise_softirq_irqoff(nr);
+ }

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