On Wed, 29 Oct 2014 08:41:13 +1100
NeilBrown <neilb@xxxxxxx> wrote:
> On Mon, 27 Oct 2014 16:27:48 -0400 Joe Lawrence <joe.lawrence@xxxxxxxxxxx>
> wrote:
>
> > Hi Neil,
> >
> > We've encountered changes in MD and mdadm that have broken our automated
> > disk removal script. In the past, we've been able to run the following
> > after a RAID1 disk component removal:
> >
> > % echo fail > /sys/block/md3/md/dev-sdr5/state
> > % echo remove > /sys/block/md3/md/dev-sdr5/state
> >
> > However, the latest RHEL6.6 code drop has rebased to sufficiently recent
> > MD kernel and mdadm changes, in which the previous commands occasionally
> > fail like so:
> >
> > * MD array is usually resyncing or checking
> > * Component disk /dev/sdr removed via HBA sysfs PCI removal
> > * Following UDEV rule fires:
> >
> > SUBSYSTEM=="block", ACTION=="remove", ENV{ID_PATH}=="?*", \
> > RUN+="/sbin/mdadm -If $name --path $env{ID_PATH}"
> >
> > % mdadm --detail /dev/md3
> > /dev/md3:
> > Version : 1.1
> > Creation Time : Tue Oct 14 17:31:59 2014
> > Raid Level : raid1
> > Array Size : 25149440 (23.98 GiB 25.75 GB)
> > Used Dev Size : 25149440 (23.98 GiB 25.75 GB)
> > Raid Devices : 2
> > Total Devices : 2
> > Persistence : Superblock is persistent
> >
> > Intent Bitmap : Internal
> >
> > Update Time : Wed Oct 15 14:22:34 2014
> > State : active, degraded
> > Active Devices : 1
> > Working Devices : 1
> > Failed Devices : 1
> > Spare Devices : 0
> >
> > Name : localhost.localdomain:3
> > UUID : 40ed68ee:ba41d4cd:28c361ed:be7470b8
> > Events : 142
> >
> > Number Major Minor RaidDevice State
> > 0 65 21 0 faulty
> > 1 65 5 1 active sync /dev/sdj5
> >
> > All attempts to remove this device fail:
> >
> > % echo remove > /sys/block/md3/md/dev-sdr5/state
> > -bash: echo: write error: Device or resource busy
> >
> > This can be traced to state_store():
> >
> > } else if (cmd_match(buf, "remove")) {
> > if (rdev->raid_disk >= 0)
> > err = -EBUSY;
> >
> > After much debugging and systemtapping, I think I've figured out that the
> > sysfs scripting may fail after the following combination of changes:
> >
> > mdadm 8af530b07fce "Enhance incremental removal."
> > kernel 30b8feb730f9 "md/raid5: avoid deadlock when raid5 array has unack
> > badblocks during md_stop_writes"
> >
> > With these two changes:
> >
> > 1 - On the user side, mdadm is trying to set the array_state to read-auto
> > on incremental removal (as invoked by UDEV rule).
> >
> > 2 - Kernel side, md_set_readonly() will set the MD_RECOVERY_FROZEN flag,
> > wake up the mddev->thread and if there is a sync_thread, it will set
> > MD_RECOVERY_INTR and then wait until the sync_thread is set to NULL.
> >
> > When md_check_recovery() gets a chance to run as part of the
> > raid1d() mddev->thread, it may or may not ever get to
> > an invocation of remove_and_add_spares(), for there are but *many*
> > conditional early exits along the way -- for example, if
> > MD_RECOVERY_FROZEN is set, the following condition will bounce out of
> > the routine:
> >
> > if (!test_and_clear_bit(MD_RECOVERY_NEEDED, &mddev->recovery) ||
> > test_bit(MD_RECOVERY_FROZEN, &mddev->recovery))
> > goto unlock;
> >
> > the next time around, MD_RECOVERY_NEEDED will have been cleared, so
> > all future tests will return 0 and the negation will always take the
> > early exit path.
> >
> > Back in md_set_readonly(), it may notice that the MD is still in use,
> > so it clears the MD_RECOVERY_FROZEN and then returns -EBUSY, without
> > setting mddev->ro. But the damage has been done as conditions have
> > been set such that md_check_recovery() will never call
> > remove_and_add_spares().
> >
> > This would also explain why an "idle" sync_action clears the wedge: it
> > sets MD_RECOVERY_NEEDED allowing md_check_recovery() to continue executing
> > to remove_and_add_spares().
> >
> > As far as I can tell, this is what is happening to prevent the "remove"
> > write to /sys/block/md3/md/dev-sdr5/state from succeeding. There are
> > certainly a lot of little bit-states between disk removal, UDEV mdadm, and
> > various MD kernel threads, so apologies if I missed an important
> > transition.
> >
> > Would you consider writing "idle" to the MD array sync_action file as a
> > safe and reasonable intermediate workaround step for our script?
> >
> > And of course, any suggestions to whether this is intended behavior (ie,
> > the removed component disk is failed, but stuck in the array)?
> >
> > This is fairly easy for us to reproduce with multiple MD arrays per disk
> > (one per partition) and interrupting a raid check on all of them
> > (especially when they are delayed waiting for the first to finish) by
> > removing the component disk via sysfs PCI removal. We can provide
> > additional debug or testing if required.
> >
>
> Hi Joe,
> thanks for the details analysis!!
>
> I think the correct fix would be that MD_RECOVERY_NEEDED should be set after
> clearing MD_RECOVERY_FROZEN, like the patch below.
> Can you confirm that it works for you?
>
> Writing 'idle' should in general be safe, so that could be used as an interim.
>
> Thanks,
> NeilBrown
>
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index c03d87b6890a..2c73fcb82593 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -5261,6 +5261,7 @@ static int md_set_readonly(struct mddev *mddev, struct block_device *bdev)
> printk("md: %s still in use.\n",mdname(mddev));
> if (did_freeze) {
> clear_bit(MD_RECOVERY_FROZEN, &mddev->recovery);
> + set_bit(MD_RECOVERY_NEEDED, &mddev->recovery);
> md_wakeup_thread(mddev->thread);
> }
> err = -EBUSY;
> @@ -5275,6 +5276,8 @@ static int md_set_readonly(struct mddev *mddev, struct block_device *bdev)
> mddev->ro = 1;
> set_disk_ro(mddev->gendisk, 1);
> clear_bit(MD_RECOVERY_FROZEN, &mddev->recovery);
> + set_bit(MD_RECOVERY_NEEDED, &mddev->recovery);
> + md_wakeup_thread(mddev->thread);
> sysfs_notify_dirent_safe(mddev->sysfs_state);
> err = 0;
> }
> @@ -5318,6 +5321,7 @@ static int do_md_stop(struct mddev *mddev, int mode,
> mutex_unlock(&mddev->open_mutex);
> if (did_freeze) {
> clear_bit(MD_RECOVERY_FROZEN, &mddev->recovery);
> + set_bit(MD_RECOVERY_NEEDED, &mddev->recovery);
> md_wakeup_thread(mddev->thread);
> }
> return -EBUSY;
Hi Neil,
In my tests, the UDEV "mdadm -If" invocation fails *and* removes the
pulled disk from the MD array. This is okay for our intentions, but I
wanted to make sure that it's okay to skip any failed-but-not-removed
state.
Tested-by: Joe Lawrence <joe.lawrence@xxxxxxxxxxx>
and should this have a
Fixes: 30b8feb730f9 ("md/raid5: avoid deadlock when raid5 array has unack badblocks during md_stop_writes")
tag to mark for stable?
Thanks,
-- Joe
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