[dm-devel] [PATCH v5 20/22] multipath -u: test if path is busy

[Hannes Reinecke]

On Sat, 14 Apr 2018 00:00:13 +0200
Martin Wilck <mwilck at suse.com> wrote:

> For "find_multipaths smart", check if a path is already in use
> before setting DM_MULTIPATH_DEVICE_PATH to 1 or 2 (and thus,
> SYSTEMD_READY=0). If we don't do this, a device which has already been
> mounted (e.g. during initrd processing) may be unmounted by systemd,
> causing havoc to the boot process.
> 
> Signed-off-by: Martin Wilck <mwilck at suse.com>
> ---
>  multipath/main.c | 39 ++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 38 insertions(+), 1 deletion(-)
> 
> diff --git a/multipath/main.c b/multipath/main.c
> index 573d94f9..c69e996e 100644
> --- a/multipath/main.c
> +++ b/multipath/main.c
> @@ -675,6 +675,9 @@ configure (struct config *conf, enum mpath_cmds
> cmd, 
>  
>  	if (cmd == CMD_VALID_PATH) {
> +		struct path *pp;
> +		int fd;
> +
>  		/* This only happens if find_multipaths and
>  		 * ignore_wwids is set, and the path is not in WWIDs
>  		 * file, not currently multipathed, and has
> @@ -682,11 +685,45 @@ configure (struct config *conf, enum mpath_cmds
> cmd,
>  		 * If there is currently a multipath device matching
>  		 * the refwwid, or there is more than one path
> matching
>  		 * the refwwid, then the path is valid */
> -		if (VECTOR_SIZE(curmp) != 0 || VECTOR_SIZE(pathvec)
> > 1)
> +		if (VECTOR_SIZE(curmp) != 0) {
> +			r = 0;
> +			goto print_valid;
> +		} else if (VECTOR_SIZE(pathvec) > 1)
>  			r = 0;
>  		else
>  			/* Use r=2 as an indication for "maybe" */
>  			r = 2;
> +
> +		/*
> +		 * If opening the path with O_EXCL fails, the path
> +		 * is in use (e.g. mounted during initramfs
> processing).
> +		 * We know that it's not used by dm-multipath.
> +		 * We may not set SYSTEMD_READY=0 on such devices, it
> +		 * might cause systemd to umount the device.
> +		 * Use O_RDONLY, because udevd would trigger another
> +		 * uevent for close-after-write.
> +		 *
> +		 * The O_EXCL check is potentially dangerous,
> because it may
> +		 * race with other tasks trying to access the
> device. Therefore
> +		 * this code is only executed if the path hasn't
> been released
> +		 * to systemd earlier (see above).
> +		 *
> +		 * get_refwwid() above stores the path we examine in
> slot 0.
> +		 */
> +		pp = VECTOR_SLOT(pathvec, 0);
> +		fd = open(udev_device_get_devnode(pp->udev),
> +			  O_RDONLY|O_EXCL);
> +		if (fd >= 0)
> +			close(fd);
> +		else {
> +			condlog(3, "%s: path %s is in use: %s",
> +				__func__, pp->dev,
> +				strerror(errno));
> +			/*
> +			 * Check if we raced with multipathd
> +			 */
> +			r
> = !sysfs_is_multipathed(VECTOR_SLOT(pathvec, 0));
> +		}
>  		goto print_valid;
>  	}
>  

But doesn't this cause udev to generate another event upon release?
Or is that prevented by the O_RDONLY setting?


Cheers,

Hannes