Messages in this thread | | | Subject | Re: [BUG] TASK_DEAD task is able to be woken up in special condition | From | Peter Zijlstra <> | Date | Fri, 06 Jan 2012 13:43:58 +0100 |
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On Fri, 2012-01-06 at 21:01 +0900, Yasunori Goto wrote:
> Do you mean the following patch?
Yes, something like that. At that point ->state should be TASK_RUNNING (since we are after all running). The unlock_wait() will synchronize against any in-progress ttwu() while its fast path is a non-atomic compare. Any ttwu after this will bail since it will either observe TASK_RUNNING or TASK_DEAD, neither are a state it will act upon.
Now the only question that remains is if we need the full memory barrier or if we can get away with less.
I guess the mb separates the write to ->state (setting TASK_RUNNING) from the read of ->pi_lock. The remote CPU must see the TASK_RUNNING, and we must see ->pi_lock taken if it is.
I also can't find anything to 'borrow' a barrier from (well I can for mainline, but not for -rt).
So yes, I guess the below will do, albeit it needs a somewhat comprehensive comment explaining its need.
Oleg, can you agree?
> --- > > Signed-off-by: Yasunori Goto <y-goto@jp.fujitsu.com> > > --- > kernel/exit.c | 4 ++++ > 1 file changed, 4 insertions(+) > > Index: linux-3.2-rc7/kernel/exit.c > =================================================================== > --- linux-3.2-rc7.orig/kernel/exit.c > +++ linux-3.2-rc7/kernel/exit.c > @@ -1038,6 +1038,10 @@ NORET_TYPE void do_exit(long code) > > preempt_disable(); > exit_rcu(); > + > + smp_mb(); > + raw_spin_unlock_wait(&tsk->pi_lock); > + > /* causes final put_task_struct in finish_task_switch(). */ > tsk->state = TASK_DEAD; > schedule(); > >
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