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dm mpath: restrict queue_if_no_path state machine

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Do not allow saving disabled queue_if_no_path if already saved as
enabled; implies multiple suspends (which shouldn't ever happen).  Log
if this unlikely scenario is ever triggered.

Also, only write MPATHF_SAVED_QUEUE_IF_NO_PATH during presuspend or if
"fail_if_no_path" message.  MPATHF_SAVED_QUEUE_IF_NO_PATH is no longer
always modified, e.g.: even if queue_if_no_path()'s save_old_value
argument wasn't set.  This just implies a bit tighter control over
the management of MPATHF_SAVED_QUEUE_IF_NO_PATH.  Side-effect is
multipath_resume() doesn't reset MPATHF_QUEUE_IF_NO_PATH unless
MPATHF_SAVED_QUEUE_IF_NO_PATH was set (during presuspend); and at that
time the MPATHF_SAVED_QUEUE_IF_NO_PATH bit gets cleared.  So
MPATHF_SAVED_QUEUE_IF_NO_PATH's use is much more narrow in scope.

Last, but not least, do _not_ disable queue_if_no_path during noflush
suspend.  There is no need/benefit to saving off queue_if_no_path via
MPATHF_SAVED_QUEUE_IF_NO_PATH and clearing MPATHF_QUEUE_IF_NO_PATH for
noflush suspend -- by avoiding this needless queue_if_no_path flag
churn there is less potential for MPATHF_QUEUE_IF_NO_PATH to get lost.
Which avoids potential for IOs to be errored back up to userspace
during DM multipath's handling of path failures.

That said, this last change papers over a reported issue concerning
request-based dm-multipath's interaction with blk-mq, relative to
suspend and resume: multipath_endio is being called _before_
multipath_resume.  This should never happen if DM suspend's
blk_mq_quiesce_queue() + dm_wait_for_completion() is genuinely waiting
for all inflight blk-mq requests to complete.  Similarly:
drivers/md/dm.c:__dm_resume() clearly calls dm_table_resume_targets()
_before_ dm_start_queue()'s blk_mq_unquiesce_queue() is called.  If
the queue isn't even restarted until after multipath_resume(); the BIG
question that still needs answering is: how can multipath_end_io beat
multipath_resume in a race!?

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
This commit is contained in:
Mike Snitzer 2020-05-27 16:32:51 -04:00
parent a862e4e215
commit 553ec94cb4
1 changed files with 28 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

34
drivers/md/dm-mpath.c
View File

@ -695,12 +695,25 @@ static int queue_if_no_path(struct multipath *m, bool queue_if_no_path,
bool save_old_value)
{
unsigned long flags;
bool queue_if_no_path_bit, saved_queue_if_no_path_bit;
spin_lock_irqsave(&m->lock, flags);
assign_bit(MPATHF_SAVED_QUEUE_IF_NO_PATH, &m->flags,
(save_old_value && test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags)) ||
(!save_old_value && queue_if_no_path));
queue_if_no_path_bit = test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags);
saved_queue_if_no_path_bit = test_bit(MPATHF_SAVED_QUEUE_IF_NO_PATH, &m->flags);
if (save_old_value) {
if (unlikely(!queue_if_no_path_bit && saved_queue_if_no_path_bit)) {
DMERR("%s: QIFNP disabled but saved as enabled, saving again loses state, not saving!",
dm_device_name(dm_table_get_md(m->ti->table)));
} else
assign_bit(MPATHF_SAVED_QUEUE_IF_NO_PATH, &m->flags, queue_if_no_path_bit);
} else if (!queue_if_no_path && saved_queue_if_no_path_bit) {
/* due to "fail_if_no_path" message, need to honor it. */
clear_bit(MPATHF_SAVED_QUEUE_IF_NO_PATH, &m->flags);
}
assign_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags, queue_if_no_path);
spin_unlock_irqrestore(&m->lock, flags);
if (!queue_if_no_path) {
@ -1653,15 +1666,18 @@ done:
}
/*
* Suspend can't complete until all the I/O is processed so if
* the last path fails we must error any remaining I/O.
* Note that if the freeze_bdev fails while suspending, the
* Suspend with flush can't complete until all the I/O is processed
* so if the last path fails we must error any remaining I/O.
* - Note that if the freeze_bdev fails while suspending, the
* queue_if_no_path state is lost - userspace should reset it.
* Otherwise, during noflush suspend, queue_if_no_path will not change.
*/
static void multipath_presuspend(struct dm_target *ti)
{
struct multipath *m = ti->private;
/* FIXME: bio-based shouldn't need to always disable queue_if_no_path */
if (m->queue_mode == DM_TYPE_BIO_BASED || !dm_noflush_suspending(m->ti))
queue_if_no_path(m, false, true);
}
@ -1683,8 +1699,10 @@ static void multipath_resume(struct dm_target *ti)
unsigned long flags;
spin_lock_irqsave(&m->lock, flags);
assign_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags,
test_bit(MPATHF_SAVED_QUEUE_IF_NO_PATH, &m->flags));
if (test_bit(MPATHF_SAVED_QUEUE_IF_NO_PATH, &m->flags)) {
set_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags);
clear_bit(MPATHF_SAVED_QUEUE_IF_NO_PATH, &m->flags);
}
spin_unlock_irqrestore(&m->lock, flags);
}