booting btrfs RAID with dracut/systemd results in open_ctree failed

[Kai Krakow <hurikhan77@gmail.com>]

Hi!

I need to boot with dracut to get my btrfs root partition properly 
initialized (because it is a multi-device btrfs). Today, after upgrading to 
systemd v220, I tracked a booting issue down to what looks like a general 
problem with the btrfs udev rules distributed with systemd:

If I drop down to an emergency shell through rd.break=pre-mount, when trying 
to mount sysroot, I get the error "open_ctree failed" and "BTRFS: failed to 
read the system array". This is generally a problem when probing for btrfs 
devices hasn't been done yet.

So I looked into the dracut sources to find that it brings it's own udev 
rule which properly does this. The caveat however is: If it already finds a 
udev rules for btrfs, it won't install its own rule. The rule in question 
is:

$ cat 64-btrfs.rules
# do not edit this file, it will be overwritten on update

SUBSYSTEM!="block", GOTO="btrfs_end"
ACTION=="remove", GOTO="btrfs_end"
ENV{ID_FS_TYPE}!="btrfs", GOTO="btrfs_end"

# let the kernel know about this btrfs filesystem, and check if it is 
complete
IMPORT{builtin}="btrfs ready $devnode"

# mark the device as not ready to be used by the system
ENV{ID_BTRFS_READY}=="0", ENV{SYSTEMD_READY}="0"

LABEL="btrfs_end"

It comes distributed with systemd so I believe this is a systemd issue. I 
fixed it by placing the following work-around:

/usr/lib/dracut/modules.d/99btrfs-device-scan/btrfs_device_scan.sh:
#!/bin/sh

type getarg >/dev/null 2>&1 || . /lib/dracut-lib.sh

info "Scanning for all btrfs devices"
/sbin/btrfs device scan >/dev/null 2>&1
/usr/lib/dracut/modules.d/99btrfs-device-scan/module-setup.sh:
#!/bin/bash

# called by dracut
check() {
    local _rootdev
    # if we don't have btrfs installed on the host system,
    # no point in trying to support it in the initramfs.
    require_binaries btrfs || return 1

    [[ $hostonly ]] || [[ $mount_needs ]] && {
        for fs in ${host_fs_types[@]}; do
            [[ "$fs" == "btrfs" ]] && return 0
        done
        return 255
    }

    return 0
}

# called by dracut
depends() {
    echo btrfs
    return 0
}

# called by dracut
install() {
    inst_hook pre-mount 99 "$moddir/btrfs_device_scan.sh"
}

This issues an explicit "btrfs device scan" in the pre-mount hook. However, 
looking at the udev rules of systemd for btrfs, it should accomblish more or 
less the same. So something is buggy or racy there. I took note that I saw 
only one of the following lines in dmesg when the problem was present:

[    5.514318] BTRFS: device label system devid 5 transid 2779055 
/dev/bcache2
[    5.514422] BTRFS: device label system devid 6 transid 2779055 
/dev/bcache1
[    5.514521] BTRFS: device label system devid 4 transid 2779055 
/dev/bcache0

Without my "fix", only one line showed up in the log - probably exactly at 
mount time when systemd's sysroot.mount unit started. It wasn't always the 
same, tho.

With v219 I only had sometimes this problem. A reboot usually fixed it. This 
supports my theory of the rule being racy somewhere, especially around the 
line ENV{ID_BTRFS_READY}=="0", ENV{SYSTEMD_READY}="0".

My btrfs setup looks like this:

Overall:
    Device size:                   2.71TiB
    Device allocated:              1.85TiB
    Device unallocated:          880.47GiB
    Device missing:                  0.00B
    Used:                          1.30TiB
    Free (estimated):              1.41TiB      (min: 1003.50GiB)
    Data ratio:                       1.00
    Metadata ratio:                   2.00
    Global reserve:              512.00MiB      (used: 0.00B)

Data,RAID0: Size:1.84TiB, Used:1.29TiB
   /dev/bcache0  628.00GiB
   /dev/bcache1  628.00GiB
   /dev/bcache2  628.00GiB

Metadata,RAID1: Size:6.00GiB, Used:4.21GiB
   /dev/bcache0    4.00GiB
   /dev/bcache1    4.00GiB
   /dev/bcache2    4.00GiB

System,RAID1: Size:32.00MiB, Used:120.00KiB
   /dev/bcache0   32.00MiB
   /dev/bcache2   32.00MiB

Unallocated:
   /dev/bcache0  293.48GiB
   /dev/bcache1  293.51GiB
   /dev/bcache2  293.48GiB

Dracut is v041, systemd is v220, kernel is 4.0.4, cmdline is:
root=/dev/bcache0 ro snd_hda_intel.enable_msi=1 rootfstype=btrfs 
rootflags=compress=lzo zswap.enabled=1 splash quiet

It may be worth noting that I'm using bcache whose udev rules may interfere 
with those for btrfs.

CC'ing bcache-devel and btrfs-devel just in case, f'up btrfs-devel.

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