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Date: Tue, 11 Sep 2018 13:44:29 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Dmitry Safonov <dima@arista.com>, linux-kernel@vger.kernel.org,
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Subject: Re: [PATCHv3 2/6] tty/ldsem: Update waiter->task before waking up
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Message-ID: <20180911114429.GT24106@hirez.programming.kicks-ass.net>
References: <20180911014821.26286-1-dima@arista.com>
 <20180911014821.26286-3-dima@arista.com>
 <20180911050449.GB4065@jagdpanzerIV>
 <20180911054129.GC4065@jagdpanzerIV>
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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.