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On 10/2/19 9:33 PM, Andrei Borzenkov wrote:
On Wed, Oct 2, 2019 at 1:19 PM Anand Jain <anand.jain@xxxxxxxxxx> wrote:
On 10/2/19 6:02 PM, Anand Jain wrote:
On 10/2/19 5:55 PM, Andrei Borzenkov wrote:
On Wed, Oct 2, 2019 at 12:27 PM Anand Jain <anand.jain@xxxxxxxxxx> wrote:
I am looking for systemd part of the answers to understand what
is triggering a strange problem. Any help is appreciated.
After mkfs.btrfs creates btrfs filesystem it scans to register the
device in btrfs.ko.
And we have 'btrfs dev scan --forget' command to undo the process of
register.
But the problem is - immediately after 'btrfs dev scan --forget' the
systemd-udevd scans the device again, defeating the purpose of the
forget as show below (scanned-by).
mkfs.btrfs -fq /dev/sdc && btrfs dev scan --forget /dev/sdc
-------------------
kernel: BTRFS: device fsid 8ea20bb2-888a-4b3b-9f4c-1db9117dc219 devid 1
transid 5 /dev/sdc scanned-by mkfs.btrfs
kernel: BTRFS: device fsid 8ea20bb2-888a-4b3b-9f4c-1db9117dc219 devid 1
transid 5 /dev/sdc scanned-by systemd-udevd
-------------------
And the problem does _not_ happen if there is a sleep of 3 secs after
the mkfs.btrfs, as below.
mkfs.btrfs -fq /dev/sdc && sleep 3 && btrfs dev scan --forget /dev/sdc
------------------
kernel: BTRFS: device fsid 601bd01a-5e6b-488a-b020-0e7556c83087 devid 1
transid 5 /dev/sdc scanned-by mkfs.btrfs
------------------
Any idea what happening from the systemd point of view?
run
udevadm monitor -ku
in both cases and post results. My educated guess is that udev scan is
in response to mkfs and you have unfortunate race condition here.
Looks like what is happening is ..
. Change in fsid (by mkfs.btrfs) notifies and triggers systemd
. Systemd checks if the device is ready by using
BTRFS_IOC_DEVICES_READY.
. However BTRFS_IOC_DEVICES_READY from systemd races with forget
command and the result depends on who wins the race.
I get this for the command mkfs.btrfs: (for /dev/sdc)
KERNEL[185263.634507] change
/devices/pci0000:5d/0000:5d:02.0/0000:65:00.0/host0/target0:2:2/0:2:2:0/block/sdc
(block)
UDEV [185263.637870] change
/devices/pci0000:5d/0000:5d:02.0/0000:65:00.0/host0/target0:2:2/0:2:2:0/block/sdc
(block)
KERNEL[185263.640572] change
/devices/pci0000:5d/0000:5d:02.0/0000:65:00.0/host0/target0:2:2/0:2:2:0/block/sdc
(block)
KERNEL[185263.641552] change
/devices/pci0000:5d/0000:5d:02.0/0000:65:00.0/host0/target0:2:2/0:2:2:0/block/sdc
(block)
UDEV [185263.644337] change
/devices/pci0000:5d/0000:5d:02.0/0000:65:00.0/host0/target0:2:2/0:2:2:0/block/sdc
(block)
UDEV [185263.647656] change
/devices/pci0000:5d/0000:5d:02.0/0000:65:00.0/host0/target0:2:2/0:2:2:0/block/sdc
(block)
And no notification for mkfs.btrfs -fq /dev/sdb
Looks like there is some rules set. But I don't find any rules
in /etc/udev/rules.d specific to /dev/sdb can it be set somewhere
else?
Default rules are in /usr/lib/udev, but rules can only block udev
events (if at all), they have no impact on what kernel does. I guess
util-linux would be a better place to ask about mkfs behavior.
In the event of a mkfs.btrfs on a device the udev action to scan
the device into the btrfs kernel is redundant !. because mkfs.btrfs
is already doing it.
/usr/lib/udev has quite a lot of rules.
Now how do I find why udev has an rule for sdc and not for sdb.
There isn't any obvious things I found in /usr/lib/udev for sdc/sdb.
Thanks, Anand
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