Re: wait_on_page_bit_common(TASK_KILLABLE, EXCLUSIVE) can miss wakeup?

[Nicholas Piggin]

Excerpts from Oleg Nesterov's message of June 30, 2020 12:02 am:
> On 06/29, Nicholas Piggin wrote:
>>
>> prepare_to_wait_event() has a pretty good pattern (and comment), I would
>> favour using that (test the signal when inserting on the waitqueue).
>>
>> @@ -1133,6 +1133,15 @@ static inline int wait_on_page_bit_common(wait_queue_head_t *q,
>>         for (;;) {
>>                 spin_lock_irq(&q->lock);
>>
>> +               if (signal_pending_state(state, current)) {
>> +                       /* Must not lose an exclusive wake up, see
>> +                        * prepare_to_wait_event comment */
>> +                       list_del_init(&wait->entry);
>> +                       spin_unlock_irq(&q->lock);
>> +                       ret = -EINTR;
> 
> Basically this is what my patch in the 1st email does. But note that we can't
> just set "ret = -EINTR" here, we will need to clear "ret" if test_and_set_bit()
> below succeeds. That is why I used another "int intr" variable.

You snipped off one more important line of context. No such games are 
required AFAIKS.

Thanks,
Nick