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

From: Kirill Tkhai <ktkhai@virtuozzo.com>
To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Dmitry Safonov <dima@arista.com>
Cc: linux-kernel@vger.kernel.org,
	"Dmitry Safonov" <0x7f454c46@gmail.com>,
	"Daniel Axtens" <dja@axtens.net>,
	"Dmitry Vyukov" <dvyukov@google.com>,
	"Michael Neuling" <mikey@neuling.org>,
	"Mikulas Patocka" <mpatocka@redhat.com>,
	"Nathan March" <nathan@gt.net>, "Pasi Kärkkäinen" <pasik@iki.fi>,
	"Peter Hurley" <peter@hurleysoftware.com>,
	"Rong, Chen" <rong.a.chen@intel.com>,
	"Tan Xiaojun" <tanxiaojun@huawei.com>,
	"Tetsuo Handa" <penguin-kernel@I-love.SAKURA.ne.jp>,
	stable@vger.kernel.org,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Jiri Slaby" <jslaby@suse.com>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: Re: [PATCHv3 2/6] tty/ldsem: Update waiter->task before waking up reader
Date: Tue, 11 Sep 2018 14:04:13 +0300	[thread overview]
Message-ID: <fe682c8e-494e-6afe-db95-83f77de55d8a@virtuozzo.com> (raw)
In-Reply-To: <20180911054129.GC4065@jagdpanzerIV>

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