linux-kernel - Re: [PATCH 2/2] sched: Introduce set_special

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Message-ID: <20180501095054.GD12235@hirez.programming.kicks-ass.net>
Date:   Tue, 1 May 2018 11:50:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     mingo@...nel.org, tglx@...utronix.de, linux-kernel@...r.kernel.org,
        will.deacon@....com, mpe@...erman.id.au, bigeasy@...utronix.de,
        gkohli@...eaurora.org, neeraju@...eaurora.org
Subject: Re: [PATCH 2/2] sched: Introduce set_special_state()

On Mon, Apr 30, 2018 at 09:40:24PM +0200, Peter Zijlstra wrote:
> On Mon, Apr 30, 2018 at 06:45:46PM +0200, Oleg Nesterov wrote:
> > On 04/30, Peter Zijlstra wrote:
> > >
> > > --- a/kernel/signal.c
> > > +++ b/kernel/signal.c
> > > @@ -1968,7 +1968,7 @@ static void ptrace_stop(int exit_code, i
> > >  	 * atomic with respect to siglock and should be done after the arch
> > >  	 * hook as siglock is released and regrabbed across it.
> > >  	 */
> > > -	set_current_state(TASK_TRACED);
> > > +	set_special_state(TASK_TRACED);
> > 
> > Yes, but please note the comment above, we need a barrier after state = TASK_TRACED,
> > that is why ptrace_stop() does set_current_state(), not __set_current_state().
> 
> OK, so I got properly lost in that stuff.
> 
> The comment says it we need to set TASK_TRACED before clearing
> JOBCTL_TRAPPING because of do_wait(), but I cannot seem to locate code
> in do_wait() and below that cares about JOBCTL_TRAPPING.
> 
> Could you elucidate my tired brain?

The only code I found that seems to care is ptrace_attach(), where we
wait for JOBCTL_TRAPPING to get cleared. That same function has a
comment about hiding the STOPPED -> RUNNING -> TRACED transition. So I'm
assuming it needs to observe TRACED if it observes !TRAPPING.

But I don't think there's enough barriers on that end to guarantee this.
Any ->state load after wait_on_bit() is, afact, free to have happened
before the ->jobctl load.