Messages in this thread | | | Date | Fri, 20 Feb 2015 19:58:00 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 6/7 linux-next] wan: cosa: replace current->state by set_current_state() |
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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 __set_current_state().
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 sleep.
In full:
current->state = TASK_UNINTERRUPTIBLE; wait = false; smp_mb(); smp_wmb(); if (wait) p->state = TASK_RUNNING; schedule();
Without that smp_mb(); the following order is possible:
if (wait) wait = false; smp_wmb(); p->state = TASK_RUNNING; current->state = TASK_UNINTERRUPTIBLE; schedule();
And we'll wait forever more..
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