linux-kernel - Re: [PATCHv3 2/6] tty/ldsem: Update waiter->task before waking up reader

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Message-ID: <20180911114429.GT24106@hirez.programming.kicks-ass.net>
Date:   Tue, 11 Sep 2018 13:44:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Dmitry Safonov <dima@...sta.com>, linux-kernel@...r.kernel.org,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Daniel Axtens <dja@...ens.net>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Michael Neuling <mikey@...ling.org>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Nathan March <nathan@...net>,
        Pasi Kärkkäinen <pasik@....fi>,
        Peter Hurley <peter@...leysoftware.com>,
        "Rong, Chen" <rong.a.chen@...el.com>,
        Tan Xiaojun <tanxiaojun@...wei.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        stable@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCHv3 2/6] tty/ldsem: Update waiter->task before waking up
 reader

On Tue, Sep 11, 2018 at 02:41:29PM +0900, Sergey Senozhatsky wrote:
> On (09/11/18 14:04), Sergey Senozhatsky wrote:
> > >  	for (;;) {
> > >  		set_current_state(TASK_UNINTERRUPTIBLE);
> > 
> > I think that set_current_state() also executes memory barrier. Just
> > because it accesses task state.
> > 
> > > -		if (!waiter.task)
> > > +		if (!READ_ONCE(waiter.task))
> > >  			break;
> > >  		if (!timeout)
> > >  			break;
> 
> This READ_ONCE(waiter.task) looks interesting. Maybe could be moved
> to a loop condition
> 
> 	while (!READ_ONCE(waiter.task)) {
> 		...
> 	}

No, it must be after set_current_state(). See that same comment.