Re: questions on wait_event ...

[Alexey Shinkin]

> 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@xxxxxxxxxxxxx
> 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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