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

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Message-ID: <fe682c8e-494e-6afe-db95-83f77de55d8a@virtuozzo.com>
Date:   Tue, 11 Sep 2018 14:04:13 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Dmitry Safonov <dima@...sta.com>
Cc:     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>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: Re: [PATCHv3 2/6] tty/ldsem: Update waiter->task before waking up
 reader

On 9/11/18 8:41 AM, 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)) {
> 		...
> 	}

We can't reorder event check and set_current_state(),
because this will lead to missing of wakeup:

	Documentation/memory-barriers.txt

Also, it looks like READ_ONCE() is not need. In case of compiler
had optimized this, then all wait_event() in kernel w/o READ_ONCE
would have not worked like expected, wouldn't they?

Kirill