[PATCH 0/1] multipathd: don't leave maps read-only after write protect is cleared

[Brian Bunker <brian@purestorage.com>]

A multipath map can stay read-only after write protect is cleared on
the storage, even though all of its paths are read-write. It happens
when the map's paths are removed and re-added while write protect is
set:

1. Write protect is set. The map is reloaded read-only.
   uev_update_path() sets force_readonly for that reload and clears it
   afterwards.

2. All of the map's paths are removed, and new paths are added while
   write protect is still set. ev_add_path() sees a read-only path and
   sets force_readonly = 1. Nothing ever clears it.

3. Write protect is cleared on the new paths, one after another. Each
   path's DISK_RO=0 uevent is ignored while any path of the map is
   still read-only.

4. The last path's uevent reloads the map. The reload sees the stale
   force_readonly = 1 and loads the map read-only, logging "reloaded
   (retval 0)".

force_readonly only works in one direction: ev_add_path() can set it
but never clears it. If the new paths had been added already writable,
ev_add_path() wouldn't have set the flag and the map wouldn't have
stuck. The paths have to be added while write protected and then
cleared in place.

After step 4 nothing reloads the map again. A rescan doesn't change the
paths' ro state, so it generates no DISK_RO uevent. And since
multipathd now sees the map as read-only, needs_ro_update() also
ignores the next DISK_RO=1 uevent for it.

If two paths' uevents happen to be handled after all paths are
writable, the map does recover: uev_update_path() clears
force_readonly after its reload, so the second reload is read-write.
That makes the problem look intermittent.

Example
-------

We hit this on RHEL 9 device-mapper-multipath 0.8.7, whose
ev_add_path() and uev_update_path() handle force_readonly the same way
as current master. The host's iSCSI sessions were torn down and
re-established while the array was write protecting the volume. From
the messages log, trimmed:

  05:48:27  ...114d8: reload [0 51200 multipath 0 1 alua 1 1
            service-time 0 6 1 135:272 1 66:624 1 69:720 1 129:560 1
            132:656 1 135:752 1]
  05:48:27  sdsd: map ...114d8 reloaded (retval 0)
  05:51:04  kernel: sd 24:0:0:48: [sdaer] Write Protect is on
  05:51:05  ...114d8: remaining active paths: 0
  05:51:07  kernel: sd 29:0:0:48: [sdaoi] Write Protect is on
            (and four more new paths)
  06:09:01  kernel: sd 24:0:0:48: [sdaer] Write Protect is off
  06:09:07  kernel: sd 29:0:0:48: [sdaoi] Write Protect is off
  06:09:10  sdaoi: update path write_protect to '0' (uevent)
  06:09:10  ...114d8: reload [0 51200 multipath 0 1 alua 1 1
            service-time 0 6 1 67:880 1 70:992 1 130:848 1 133:960 1
            65:1072 1 67:1056 1]
  06:09:10  sdaoi: map ...114d8 reloaded (retval 0)

The state afterwards, as recorded by the test:

  dm-106: ro
  sdaer: rw  sdagu: rw  sdaix: rw  sdala: rw  sdand: rw  sdaoi: rw

The patch
---------

The patch adds map_has_ro_path(), which checks whether any of a map's
paths is read-only in sysfs, and uses it wherever force_readonly is
set. ev_add_path() now computes the flag from all of the map's paths
and clears it once the map is loaded, as uev_update_path() already
does after its reload. uev_update_path() and update_map() recompute it
before reloading. With the patch, the 06:09:10 reload would have found
no read-only path and loaded the map read-write.

Open points for review:

- uev_update_path() now takes the read-only state from sysfs rather
  than from the uevent's DISK_RO value. That relies on set_disk_ro()
  updating the disk's read-only state before sending the uevent, which
  current kernels do. Older kernels (e.g. v4.18) sent the uevent first.

- force_readonly is a one-shot hint that several callers have to set
  and clear correctly. A follow-up could remove it and have
  dm_addmap_create() and dm_addmap_reload() call map_has_ro_path()
  themselves. That would move sysfs_get_ro() into libmultipath and
  change the multipath command's behavior too, so it isn't part of
  this fix.

Brian Bunker (1):
  multipathd: don't leave maps read-only after write protect is cleared

 multipathd/main.c | 44 +++++++++++++++++++++++++++++++++++++-------
 1 file changed, 37 insertions(+), 7 deletions(-)

-- 
2.56.0