About monitor raid event using poll/select

I recently did a project which monitor raids event in system. I used this way:

while(1){
	open(/proc/mdstat);
	poll();
    do-something
	close
}

I wanted to monitor all events in times.Supposing events A--10s-->B---1hour---->C,do-somethings taks 30s. First I must fetch the event A.But I do do-somthing using 30s,so I did not monitor event B. But the interval of B and C is 1 hour,so I only after 1 hours can detected the events B.

I can modified the do-something in order to reduce times.But in theory,I can omit event. so I think modified kernel function mdstat_poll:

index 39b27c4..aaecc03 100644
@@ -6539,8 +6539,10 @@ static unsigned int mdstat_poll(struct file *filp, poll_table *wait)
        /* always allow read */
        mask = POLLIN | POLLRDNORM;
 
-   if (mi->event != atomic_read(&md_event_count))
+ if (mi->event != atomic_read(&md_event_count)){
+              mi->event = atomic_read(&md_event_count);
                mask |= POLLERR | POLLPRI;
+ }
        return mask;
 }
 user space function like this:

opne(/proc/mdstat)
while(1){
poll
do-something
}
close

2011-06-02

majianpeng

This is the wrong way to use 'poll' - no matter what sort of file descriptor you have.

You need to open /proc/mdstat and keep it open.

You then read /proc/mdstat and possibly act upon the state that you find there.
Then you call 'poll'.
When poll tells you that something has happened, you seek back to the start of the file and read it again. If something has changed, you can act on that change.
Then you can call 'poll' again.

'poll' tells you if something has changed since the last time you read the file.
When you open the file and use poll straight away it should tell you that the file has data ready for you.. maybe it doesn't but it should. Because there is data there for you to read.

NeilBrown