Re: [PATCH 1/6] common: udev settle before _scratch_pool_mkfs

[David Sterba <dsterba@suse.cz>]

On Sat, May 18, 2024 at 05:57:12AM +0800, Anand Jain wrote:
> On 5/17/24 06:12, David Sterba wrote:
> > From: Josef Bacik <josef@toxicpanda.com>
> > 
> > There are some btrfs tests that do _scratch_pool_mkfs in a loop.
> > Sometimes this fails with EBUSY.  Tracing revealed that udevd will
> > sometimes write to /sys/block/device/uevent to make sure an event
> > triggers to rules get written.  However these events will not get sent
> > to user space until after an O_EXCL open as been closed.  The general
> > flow is something like
> > 
> > mkfs.btrfs /dev/sda /dev/sdb /dev/sdc /dev/sdd
> > mount /dev/sda /mnt/test
> > <things>
> > umount /mnt/test
> > 
> > in a loop.  The problem is udevd will add uevents for the devices and
> > they won't get delivered until after the umount.  If we're doing the
> > above sequence in a loop the next mkfs.btrfs will fail because udev is
> > touching the devices to consume the KOBJ_CHANGE event.
> > 
> > Fix this by doing a udev settle before _scratch_pool_mkfs.
> > 
> 
> 
> 
> > Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> > Signed-off-by: David Sterba <dsterba@suse.com>
> > ---
> >   common/rc | 6 ++++++
> >   1 file changed, 6 insertions(+)
> > 
> > diff --git a/common/rc b/common/rc
> > index 0fe56382a6a497..5d38571ffe87eb 100644
> > --- a/common/rc
> > +++ b/common/rc
> > @@ -903,6 +903,12 @@ _scratch_pool_mkfs()
> >   {
> >       case $FSTYP in
> >       btrfs)
> > +	# For multi-disk file systems udev can queue up events on the device
> > +	# when we mkfs the device, and thus tie up the device after we've
> > +	# unmounted.  Tests that _scratch_pool_mkfs() in a loop can sometimes
> > +	# trip over udev trying to do the updates after the umount, so make sure
> > +	# we settle before we try mkfs'ing so we don't get an EBUSY
> > +	$UDEV_SETTLE_PROG >/dev/null 2>&1
> >           $MKFS_BTRFS_PROG $MKFS_OPTIONS $* $SCRATCH_DEV_POOL > /dev/null
> >           ;;
> 
> 
> Just curious: have we seen this issue even after the btrfs-progs commit 
> below?
> 
>    e54514aaeab6 btrfs-progs: fix stray fd close in open_ctree_fs_info()

That's a good question, I don't know but I can revert the change and see
if things break. This may take time as triggering the udev/mkfs race is
not reliable.