Merge branch 'md-suspend-rewrite' into md-next

From Yu Kuai, written by Song Liu

Recent tests with raid10 revealed many issues with the following scenarios:

- add or remove disks to the array
- issue io to the array

At first, we fixed each problem independently respect that io can
concurrent with array reconfiguration. However, with more issues reported
continuously, I am hoping to fix these problems thoroughly.

Refer to how block layer protect io with queue reconfiguration (for
example, change elevator):

blk_mq_freeze_queue
-> wait for all io to be done, and prevent new io to be dispatched
// reconfiguration
blk_mq_unfreeze_queue

I think we can do something similar to synchronize io with array
reconfiguration.

Current synchronization works as the following. For the reconfiguration
operation:

1. Hold 'reconfig_mutex';
2. Check that rdev can be added/removed, one condition is that there is no
   IO (for example, check nr_pending).
3. Do the actual operations to add/remove a rdev, one procedure is
   set/clear a pointer to rdev.
4. Check if there is still no IO on this rdev, if not, revert the
   change.

IO path uses rcu_read_lock/unlock() to access rdev.

- rcu is used wrongly;
- There are lots of places involved that old rdev can be read, however,
many places doesn't handle old value correctly;
- Between step 3 and 4, if new io is dispatched, NULL will be read for
the rdev, and data will be lost if step 4 failed.

The new synchronization is similar to blk_mq_freeze_queue(). To add or
remove disk:

1. Suspend the array, that is, stop new IO from being dispatched
   and wait for inflight IO to finish.
2. Add or remove rdevs to array;
3. Resume the array;

IO path doesn't need to change for now, and all rcu implementation can
be removed.

Then main work is divided into 3 steps:

First, first make sure new apis to suspend the array is general:

- make sure suspend array will wait for io to be done(Done by [1]);
- make sure suspend array can be called for all personalities(Done by [2]);
- make sure suspend array can be called at any time(Done by [3]);
- make sure suspend array doesn't rely on 'reconfig_mutex'(PATCH 3-5);

Second replace old apis with new apis(PATCH 6-16). Specifically, the
synchronization is changed from:

  lock reconfig_mutex
  suspend array
  make changes
  resume array
  unlock reconfig_mutex

to:
   suspend array
   lock reconfig_mutex
   make changes
   unlock reconfig_mutex
   resume array

Finally, for the remain path that involved reconfiguration, suspend the
array first(PATCH 11,12, [4] and PATCH 17):

Preparatory work:
[1] https://lore.kernel.org/all/20230621165110.1498313-1-yukuai1@huaweicloud.com/
[2] https://lore.kernel.org/all/20230628012931.88911-2-yukuai1@huaweicloud.com/
[3] https://lore.kernel.org/all/20230825030956.1527023-1-yukuai1@huaweicloud.com/
[4] https://lore.kernel.org/all/20230825031622.1530464-1-yukuai1@huaweicloud.com/

* md-suspend-rewrite:
  md: rename __mddev_suspend/resume() back to mddev_suspend/resume()
  md: remove old apis to suspend the array
  md: suspend array in md_start_sync() if array need reconfiguration
  md/raid5: replace suspend with quiesce() callback
  md/md-linear: cleanup linear_add()
  md: cleanup mddev_create/destroy_serial_pool()
  md: use new apis to suspend array before mddev_create/destroy_serial_pool
  md: use new apis to suspend array for ioctls involed array reconfiguration
  md: use new apis to suspend array for adding/removing rdev from state_store()
  md: use new apis to suspend array for sysfs apis
  md/raid5: use new apis to suspend array
  md/raid5-cache: use new apis to suspend array
  md/md-bitmap: use new apis to suspend array for location_store()
  md/dm-raid: use new apis to suspend array
  md: add new helpers to suspend/resume and lock/unlock array
  md: add new helpers to suspend/resume array
  md: replace is_md_suspended() with 'mddev->suspended' in md_check_recovery()
  md/raid5-cache: use READ_ONCE/WRITE_ONCE for 'conf->log'
  md: use READ_ONCE/WRITE_ONCE for 'suspend_lo' and 'suspend_hi'

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