Messages in this thread | | | Date | Wed, 18 Jan 2012 15:20:05 +0100 | From | Oleg Nesterov <> | Subject | Re: [BUG] TASK_DEAD task is able to be woken up in special condition |
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On 01/18, Ingo Molnar wrote: > > Well, but does the -rt kernel suffer from the same race?
I think yes.
To remind, the problem is generic, it is not bound to this particular place. Let me repeat:
Suppose that the task T does
set_current_state(TASK_INTERRUPTIBLE);
set_current_state(TASK_UNINTERRUPTIBLE); schedule();
try_to_wake_up(TASK_UNINTERRUPTIBLE) in between can observe this task in TASK_INTERRUPTIBLE state. Then it can set RUNNING/WAKING after T sets ->state = TASK_UNINTERRUPTIBLE.
For example, this is possibly if T simply does wait_event() twice when the the 1st wait_event() doesn't sleep.
do_exit() is different because it can not handle the spurious wakeup. Well, may be we can? we can simply do
for (;;) { tsk->state = TASK_DEAD; schedule(); }
__schedule() can't race with ttwu() once it takes rq->lock. If the exiting task is deactivated, finish_task_switch() will see EXIT_DEAD. Unless I missed something, the only problem is preempt_disable(), but schedule_debug() checks ->exit_state.
OTOH, if we fix this race then probably schedule_debug() should check state == EXIT_DEAD instead.
> So if there's something in the -rt kernel that fixes this race > we'd like to have that. If the bug is present in the -rt kernel > then why didn't it ever get triggered? We caught much more > narrow races in -rt, and very early on in the project.
I do not know.
Oleg.
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