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