Re: [PATCH 6/7 linux-next] wan: cosa: replace current->state by set_current_state()

From: Peter Zijlstra
Date: Fri Feb 20 2015 - 13:58:16 EST

On Fri, Feb 20, 2015 at 09:34:28PM +0300, Sergei Shtylyov wrote:
> Hello.
> On 02/20/2015 09:12 PM, Fabian Frederick wrote:
> >Use helper functions to access current->state.
> >Direct assignments are prone to races and therefore buggy.
> >current->state = TASK_RUNNING is replaced by __set_current_state()
> You sometimes use __set_current_state() and sometimes set_current_state().

It depends on which state; setting yourself TASK_RUNNING is free of
wakeup races -- you're already running after all, so it can safely use

Setting a blocking state otoh needs set_current_state() which issues a
full memory barriers with the store (critically in this case,
effectively after the store) such that it orders the state store with a
subsequent load in the condition check if it really needs to go to

In full:

current->state = TASK_UNINTERRUPTIBLE; wait = false;
smp_mb(); smp_wmb();
if (wait) p->state = TASK_RUNNING;

Without that smp_mb(); the following order is possible:

if (wait)
wait = false;
p->state = TASK_RUNNING;
current->state = TASK_UNINTERRUPTIBLE;

And we'll wait forever more..

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