linux-kernel - Re: [PATCH 6/7 linux-next] wan: cosa: replace current->state by set_current

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Date:	Sat, 21 Feb 2015 08:42:23 +0100 (CET)
From:	Fabian Frederick <fabf@...net.be>
To:	Peter Zijlstra <peterz@...radead.org>,
	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Jan \"Yenya\" Kasprzak" <kas@...muni.cz>
Subject: Re: [PATCH 6/7 linux-next] wan: cosa: replace current->state by
 set_current_state()



> On 20 February 2015 at 19:58 Peter Zijlstra <peterz@...radead.org> wrote:
>
>
> 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..
Do I have to add more comments in changelogs or is it OK for you ?
Maybe something like:
"
current->state = TASK_RUNNING can safely be converted to __set_current_state()
as we're already in that state. Other assignments are converted to
set_current_state() (which uses set_mb()).

"
Regards,
Fabian
>
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