[systemd-devel] multipath breaks with recent udev/systemd

[Alexander E. Patrakov]

15.12.2014 14:31, Hannes Reinecke wrote:
> Hi all,
>
> in commit 3ebdb81ef088afd3b4c72b516beb5610f8c93a0d
> (udev: serialize/synchronize block device event handling with file
> locks) udev started using flock() on the device node, supposedly to
> synchronize with an ominous 'block event handling'.

This is a duplicate of the issue that I have reported earlier:

https://www.redhat.com/archives/dm-devel/2014-October/msg00210.html

Therefore, I want to be CCed on replies.

>
> The code looks like this:
>
>                    if (d) {
>                          fd_lock = open(udev_device_get_devnode(d),
> O_RDONLY|O_CLOEXEC|O_NOFOLLOW|O_NONBLOCK);
>                          if (fd_lock >= 0 && flock(fd_lock,
> LOCK_SH|LOCK_NB) < 0) {
>                               log_debug("Unable to flock(%s),
> skipping event handling: %m", udev_device_get_devnode(d));
>                               err = -EWOULDBLOCK;
>                               goto skip;
>                         }
>                     }
>
> However, multipath since several years is using a similar construct
> to lock all devices belonging to a multipath device table before
> creating a mulitpath dm-device:
>
> 	vector_foreach_slot (mpp->pg, pgp, i) {
> 		if (!pgp->paths)
> 			continue;
> 		vector_foreach_slot(pgp->paths, pp, j) {
> 			if (lock && flock(pp->fd, LOCK_SH | LOCK_NB) &&
> 			    errno == EWOULDBLOCK)
> 				goto fail;
> 			else if (!lock)
> 				flock(pp->fd, LOCK_UN);
> 		}
> 	}
>
> So during bootup it's anyone's guess who's first, multipath or udev.
> And depending on the timing either multipath will fail to setup
> the device-mapper device or udev will simply ignore the device.
> Neither of those is a good, but the first is an absolute killer for
> modern systems which _rely_ on udev to configure devices.
>
> So how it this supposed to work?
> Why does udev ignore the entire event if it can't get the lock?
> Shouldn't it rather be retried?
> What is the supposed recovery here?
>
> Cheers,
>
> Hannes
>


-- 
Alexander E. Patrakov