Re: [MDADM PATCH 2/2] Give enough time to udev to handle events

[Xiao Ni <xni@redhat.com>]

----- Original Message -----
> From: "NeilBrown" <neilb@suse.com>
> To: "Xiao Ni" <xni@redhat.com>, linux-raid@vger.kernel.org
> Cc: "jes sorensen" <jes.sorensen@gmail.com>
> Sent: Tuesday, September 19, 2017 2:49:19 PM
> Subject: Re: [MDADM PATCH 2/2] Give enough time to udev to handle events
> 
> On Tue, Sep 19 2017, Xiao Ni wrote:
> 
> > After mdadm -S /dev/md0, the device node /dev/md0 still exists. The Remove
> > events are generated by md_free() -> del_gendisk() ->
> > blk_unregister_queue().
> > After calling close(mdfd) the Remove events is generated. We should give
> > udev
> > a little time to handle the event.
> >
> > I tried usleep(100*1000), but the problem still can be reproduced sometime.
> > So I choose to sleep(1). Because after close(mdfd) it can be generated
> > CHANGE
> > events too. So it's ok to choose to sleep(1) to wait udev to handle CHANGE
> > events.
> 
> I really don't like this approach.  The fact that 1 second works for you
> is no guarantee that it will work for everybody.
> We have a few sleeps in the code already, but I don't like them either.
> Let's try not to add more.
Hi Neil

Yes, there are already some sleeps during STOP_ARRAY
        count = 25; err = 0;
        while (count && fd >= 0 &&
               (err = ioctl(fd, STOP_ARRAY, NULL)) < 0 && errno == EBUSY) {
                usleep(200000);
                count --;
        }
But it doesn't give time to wait for handling udev event which is generated
by close(mdfd). Because they are asynchronous, so there is a possibility that
the device node still exist after mdadm command exist. 

> 
> If there is some event that you want to wait for, wait for that event.
> e.g. if you want to wait for /dev/md0 to disappear then write a loop:
> 
> while /dev/md0 exists
>    usleep(1000)

Ok, I'll try this.

> 
> But I'm still not convinced that this is really needed.  If it is, then
> maybe some sort of kernel fix would be better.
> 

Hmm the reason I focus on this is that there are some bugs about this.
After some investigation I know it's that we don't wait for udev remove 
event. Because the event is generated by mdadm, so we should guarantee 
the udev event finishes, right?
Do you have a better solution for this? The REMOVE event is created by 
close(mdfd). I don't know how to wait for remove event in kernel space.

Best Regards
Xiao