Messages in this thread Patch in this message | | | Subject | [PATCH -rt] Fix TASKLET_STATE_SCHED WARN_ON() | From | john stultz <> | Date | Tue, 05 Jun 2007 19:17:23 -0700 |
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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: http://lkml.org/lkml/2007/2/25/1
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 __tasklet_common_schedule().
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?
thanks -john
Signed-off-by: John Stultz <johnstul@us.ibm.com>
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); + } tasklet_unlock(t); } }
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