Re: questions on wait_event ...

["Alexey Shinkin" <alexshinkin@hotmail.com>]

>Alexey Shinkin wrote:
>>Hi , all !
>>Could anyone please clarify one thing in that old well known
>>wait_event_... code (taken from 2.6.5 wait.h ):
>>
>>#define __wait_event_interruptible(wq, condition, ret)          \
>>do {                                                                    \
>>        wait_queue_t __wait;                                     \
>>        init_waitqueue_entry(&__wait, current);           \
>>                                                                        \
>>        add_wait_queue(&wq, &__wait);                   \
>>        for (;;) {                                                      \
>>                set_current_state(TASK_INTERRUPTIBLE);    \
>>                if (condition)                                          \
>>                        break;
>>........................................
>>
>>Is it possible that scheduling happen after set_current_state() but before
>>checking the condition ?
>>If yes - even if we will have condition==TRUE by this moment - the 
>>scheduler
>>will make the process to sleep anyway , right ?
>
>Yes, but since the condition would then have changed after we were put into 
>the wait queue, they would have woken up the queue and we should be woken 
>up again.
>
>--
>Robert Hancock      Saskatoon, SK, Canada
>To email, remove "nospam" from hancockr@nospamshaw.ca
>Home Page: http://www.roberthancock.com/
>


And what if the condition have changed after we have checked it in 
wait_event() but
before calling __wait_event() and before putting the process into the wait 
queue ?
The process could not be woken up "in advance" , right ?


#define wait_event(wq, condition)        \
do {                                                   \
        if (condition)                                \
                break;                                 \
   /* and here we have condition changed  ???? */
        __wait_event(wq, condition);        \
} while (0)


Regards
Alexey Shinkin

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