Re: basic and stupid question on wait_event and wake_up

Dear Domen,
Thanks for your reply first. 

I understand it this way:
- condition
  Just checking the condition is one way (if you don't have a wake_up
  source, like an interrupt), but that's not really what wait_event does.
  It would be something like
while (condition) {
	msleep(10);
}
  There was some talk on poll_wait(), but I don't know what happened to
  it.

So you mean in my senario (wake the process up in the interrupt handler), I 
needn't to use wake_up at all? A "condition == true" in the interrupt 
handler is enough to wake the sleeping process up? Am I right?

I checked the source code in linux/wait.h and here is the defination of 
wait_event:

#define __wait_event(wq, condition) 					
do {									DEFINE_WAIT(__wait);						
	for (;;) {							\
		prepare_to_wait(&wq, &__wait, TASK_UNINTERRUPTIBLE);	
                     if (condition)				
			break;					
		schedule();					
	}							
	finish_wait(&wq, &__wait);				
} while (0)

#define wait_event(wq, condition) 					
do {								
	if (condition)	 					
		break;						
	__wait_event(wq, condition);				
} while (0)

From the source code, it seems like that this mechanism doesn't use 

msleep(), like what you mentioned, to release its executing. Instead, it 
uses schedule() to do that. 

- wake_up
  Just wake_up isn't enough, you get a race:
  |   interrupt handler   |   process      |
  ------------------------------------------
  |   do_something()      |                |
  |   wake_up()           |                |
  |   ...                 |   wait on wq   |

  And so you have a process waiting on waitqueue, that just missed the
  wakeup. Obviously should not be used.

- wake_up & condition
  |   interrupt handler   |   process      |
  ------------------------------------------
  |   flag = 1            |                |
  |   wake_up()           |                |
  |   ...                 |   wait_event   |
  |   ...                 |   flag = 0     |

  This will work properly and if wait_event misses a wake_up, the
  condition check (flag)  will kick in before putting it to sleep.

Thanks for your explaining on the race problem. I can understand this now. 
However I still cannot understand, is such a problem: In the above figures 
for my case, if flag=1 could wake the process up, then what's the use of 
wake_up()? From my understanding if the condition turns true, then the 
process which depends on this condition will be waken up. Thus what's the 
exact use of wake_up()? Also in my program, I tried to remove wake_up() 
sentence and it seems that there is no difference on the result.

Thanks for the explanation.

BR
Ming

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