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

[Kirill Tkhai]

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