basic and stupid question on wait_event and wake_up
Domen Puncer
domen.puncer at telargo.com
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) {
msleep(10);
}
There was some talk on poll_wait(), but I don't know what happened to
it.
- 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.
Domen
>
> 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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