Messages in this thread | | | From | (Eric W. Biederman) | Subject | Re: /proc dcache deadlock in do_exit | Date | Tue, 27 Nov 2007 18:53:29 -0700 |
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Andrea Arcangeli <andrea@suse.de> writes:
> On Tue, Nov 27, 2007 at 02:38:52PM -0800, Andrew Morton wrote: >> I don't see why the schedule() will not return? Because the task has >> PF_EXITING set? Doesn't TASK_DEAD do that? > > Ouch, I assumed you couldn't sleep safely anymore in release_task > given it's the function that will free the task structure itself and > there was no preempt related action anywhere close to it! > delayed_put_task_struct can be called if a quiescent point is reached > and any scheduling would exactly allow it to run (it requires quite a > bit of a race, with local irq triggering a reschedule and the timer > irq invoking the tasklet to run to free the task struct before do_exit > finishes and all other cpus in quiescent state too). > > So a corollary question is how can it be safe to call > preempt_disable() after call_rcu(delayed_put_task_struct)?
> Back in sles9 preempt_disable was implemented as > _raw_write_unlock(&tasklist_lock) and it happened _before_ > release_task, and scheduling there wouldn't return because PF_DEAD was > already set. If mainline can come back, it will crash for a different > reason because the task struct is long gone by the time > release_task+schedule() runs. Either ways, still a kernel crashing bug > there is. Or is there some magic that prevents call_rcu + schedule to > invoke the rcu callback?
I don't quite see where it comes from but there is another reference on the task struct held by the scheduler That we don't drop until finish_task_switch with exit_state == TASK_DEAD.
So since we have one additional reference the task_struct won't get freed.
We don't set TASK_DEAD until much later.
> So you may need to apply this one too (this one is needed to fix the > second bug, my previous patch is needed after applying this one):
No. We should be fine.
In fact it looks like this is just a sles9 issue and mainline is fine, as we can safely schedule until just before the end of do_exit.
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