Re: [PATCH] generic/492: bypass the blkid cache when probing the label

[Baokun Li <libaokun@linux.alibaba.com>]

Hi Darrick,

On 2026/9/21 13:27, Darrick J. Wong wrote:
> On Sun, Sep 20, 2026 at 05:41:37PM +0800, Baokun Li wrote:
>> blkid may serve a stale label from its cache instead of probing the
>> device, so the label just set via xfs_io can be missed and the output
>> mismatch is non-deterministic.
>>
>> Pass "-c /dev/null" so that blkid always probes the on-disk superblock
>> directly.  Unlike the low-level probing mode (-p), which requires
>> util-linux 2.16 or later, this is also supported by the legacy blkid
>> shipped with e2fsprogs.
>>
>> Fixes: c3c9630968a6 ("generic: test online label ioctl")
>> Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
> Seems fine to me; I guess blkid could be racing with whatever updates
> the cache...udev?


Not a race, and not related to udev.  It's a long story; details below.

In the xfstests-bld appliance blkid is e2fsprogs' legacy blkid and its
cache is /etc/blkid.tab; the only writers are blkid itself and
e2fsprogs' fsck (both probe with the cache enabled).  systemd's udev
uses libblkid's low-level probe API and never touches this file.

What we hit is a strictly sequential sequence that depends on two
"same second" coincidences.  The cache validity check (blkid_verify()
in lib/blkid/probe.c) is:

    now >= TIME && mtime(seconds) <= TIME && age < 2s

Both sides are in seconds, and only the 2s window applies to a fresh
blkid process (BLKID_BID_FL_VERIFIED is not persistent).

mkfs only advances the device mtime and never updates the cache; the fs
label ioctl updates neither, so right after setting a new label a
cached lookup can still return the old one.

The trap is set by _check_scratch_fs: it runs "fsck -t ext4 ...", which
probes the device with the cache enabled and writes the entry back with
TIME=now and the current label state.

Here is a sequential 490/491/492 run that trips it (timestamps from a
CI log, ext4/bigalloc_1k):

    M-2  09:09:44  end of 490: fsck finds the previous entry stale
                   (age >= 2s) -> probes -> tab = {no LABEL, TIME=44}

    M-1  09:09:45  start of 491: mkfs -> device mtime = 45
                   (mkfs doesn't update the cache; tab stays TIME=44)
    M    09:09:46  end of 491: unmount + _check_scratch_fs
                   -> fsck sees mtime(45) > TIME(44) -> probes
                   -> tab = {no LABEL, TIME=46}          <- trap

    M    09:09:46  start of 492: mkfs -> device mtime = 46
    M    09:09:46  mount; ioctl label="label.492" (mtime unchanged)
    M    09:09:46  blkid(35): mtime(46) <= TIME(46), age < 2s
                   -> cache deemed valid, no probe -> prints no LABEL
    M+1  09:09:47  umount; blkid(39): still < 2s -> again
    M+1  09:09:47  check reports output mismatch (two lines missing)

The difference between the e2fsprogs and util-linux libblkid is the
time granularity: the former stores seconds only (TIME="%ld",
bid_time = time(0), compares st_mtime <= bid_time), while the latter
stores usec (TIME="sec.usec") and has an nsec tie-break in the compare,
so the same-second case is much easier to hit with the e2fsprogs blkid.

The patch avoids this by probing with -c /dev/null (the legacy blkid
has no -p).  Reproduced on ext4/bigalloc_1k: unpatched 10 of 65 runs
fail, patched 120/120 pass.


>
> Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
>
> --D


Thanks for the review!

Cheers,
Baokun