dm-mpath: Don't grab work_mutex while probing paths

Grabbing the work_mutex keeps probe_active_paths() from running at the
same time as multipath_message(). The only messages that could interfere
with probing the paths are "disable_group", "enable_group", and
"switch_group". These messages could force multipath to pick a new
pathgroup while probe_active_paths() was probing the current pathgroup.
If the multipath device has a hardware handler, and it switches active
pathgroups while there is outstanding IO to a path device, it's possible
that IO to the path will fail, even if the path would be usable if it
was in the active pathgroup. To avoid this, do not clear the current
pathgroup for the *_group messages while probe_active_paths() is
running. Instead set a flag, and probe_active_paths() will clear the
current pathgroup when it finishes probing the paths. For this to work
correctly, multipath needs to check current_pg before next_pg in
choose_pgpath(), but before this patch next_pg was only ever set when
current_pg was cleared, so this doesn't change the current behavior when
paths aren't being probed. Even with this change, it is still possible
to switch pathgroups while the probe is running, but only if all the
paths have failed, and the probe function will skip them as well in this
case.

If multiple DM_MPATH_PROBE_PATHS requests are received at once, there is
no point in repeatedly issuing test IOs. Instead, the later probes
should wait for the current probe to complete. If current pathgroup is
still the same as the one that was just checked, the other probes should
skip probing and just check the number of valid paths.  Finally, probing
the paths should quit early if the multipath device is trying to
suspend, instead of continuing to issue test IOs, delaying the suspend.

While this patch will not change the behavior of existing multipath
users which don't use the DM_MPATH_PROBE_PATHS ioctl, when that ioctl
is used, the behavior of the "disable_group", "enable_group", and
"switch_group" messages can change subtly. When these messages return,
the next IO to the multipath device will no longer be guaranteed to
choose a new pathgroup. Instead, choosing a new pathgroup could be
delayed by an in-progress DM_MPATH_PROBE_PATHS ioctl. The userspace
multipath tools make no assumptions about what will happen to IOs after
sending these messages, so this change will not effect already released
versions of them, even if the DM_MPATH_PROBE_PATHS ioctl is run
alongside them.

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

0 files changed, 0 insertions, 0 deletions