basic and stupid question on wait_event and wake_up

Domen Puncer domen.puncer at
Mon Aug 13 17:54:23 EST 2007

On 12/08/07 13:57 +0000, Ming Liu wrote:
> Dear all,
> I am reading LDD(V3) chapter 6 on the topic of wait_event(queue, condition) 
> and wake_up(queue) functions. I am quite confused on the sayings. One is 
> "Until condition evaluates to a true value, the process continues to 
> sleep", which looks like that 'condition' is the one who wake up the 
> process from its sleeping. However the other saying is "The basic function 
> that wakes up sleeping processes is called wake_up, and wake_up wakes up 
> all processes waiting on the given queue" So who is the exact one to wake 
> the sleeping process up at all, condition or wake_up? From my 
> understanding, if the condition becomes true, then the sleeping process 
> will leave its sleeping status and wake up. Then what's the use of wake_up 
> function? 

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) {
  There was some talk on poll_wait(), but I don't know what happened to

- 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.

> My senario could be described as: in my char device driver, I use one ioctl 
> command to initiate a DMA transfer. After all related registers are 
> initiated, this process will be put to sleep for saving CPU cycles. In the 
> interrupt handler which is for a DMA_done, I wake that process up and 
> resume its following executing. With this method, in my application program 
> if I release a DMA initiation command, it is a Blocking operation and it 
> will wait until the DMA transfer is done. 

Looks like you should use the "wake_up and condition" option.


> Perhaps my question is quite simple or basic. Thanks for any explanation 
> and comment on this topic in my senario from you experts. Thanks a lot.
> Br
> Ming
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