Re: [PATCH 1/1] do_exit(): Solve possibility of BUG() due to race with try_to_wake_up()

From: Peter Zijlstra
Date: Wed Sep 03 2014 - 10:44:59 EST

On Wed, Sep 03, 2014 at 03:36:40PM +0200, Oleg Nesterov wrote:
> Peter, sorry for slow responses.

No worries, I'm not entirely fast myself. Slept most of the day :-)

> Ah, I simply do not know what is cheaper, even on x86. Well, we need
> to enable/disable irqs, but again I do not really know how much does
> this cost.

Ah good point about that IRQ thing, yes that's horribly expensive.

> I can even say what (imo) looks better, lock/unlock above or
> // Ensure that the previous __set_current_state(RUNNING) can't
> // leak after spin_unlock_wait()
> smp_mb();
> spin_unlock_wait();
> // Another mb to ensure this too can't be reordered with unlock_wait
> set_current_state(TASK_DEAD);
> What do you think looks better?

spin_unlock_wait() would be a control dependency right? Therefore that
store could not creep up anyhow.
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