linux-kernel - Re: [RFC][PATCH] sched: Better document ttwu()

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Message-ID: <20200703113603.GL117543@hirez.programming.kicks-ass.net>
Date:   Fri, 3 Jul 2020 13:36:03 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        vincent.guittot@...aro.org, mgorman@...e.de,
        Oleg Nesterov <oleg@...hat.com>, david@...morbit.com
Subject: Re: [RFC][PATCH] sched: Better document ttwu()

On Fri, Jul 03, 2020 at 10:30:12AM +0200, Peter Zijlstra wrote:
> On Thu, Jul 02, 2020 at 07:39:16PM +0100, Valentin Schneider wrote:
> > > @@ -3134,8 +3274,12 @@ static inline void prepare_task(struct task_struct *next)
> > >       /*
> > >        * Claim the task as running, we do this before switching to it
> > >        * such that any running task will have this set.
> > > +	 *
> > > +	 * __schedule()'s rq->lock and smp_mb__after_spin_lock() orders this
> > > +	 * store against prior state change of @next, also see
> > > +	 * try_to_wake_up(), specifically smp_load_acquire(&p->on_cpu).
> > 
> > smp_*cond*_load_acquire(&p->on_cpu, <blah>)
> 
> Both, but yeah.. arguably the cond one is the more important one.

Ah no, this one really want to match the WF_ON_CPU case. I'll clarify
non-the-less.