deadlock between spa_errlog_lock and dp_config_rwlock

There is a lock order inversion deadlock between `spa_errlog_lock` and
`dp_config_rwlock`:

A thread in `spa_delete_dataset_errlog()` is running from a sync task.
It is holding the `dp_config_rwlock` for writer (see
`dsl_sync_task_sync()`), and waiting for the `spa_errlog_lock`.

A thread in `dsl_pool_config_enter()` is holding the `spa_errlog_lock`
(see `spa_get_errlog_size()`) and waiting for the `dp_config_rwlock` (as
reader).

Note that this was introduced by #12812.

This commit address this by defining the lock ordering to be
dp_config_rwlock first, then spa_errlog_lock / spa_errlist_lock.
spa_get_errlog() and spa_get_errlog_size() can acquire the locks in this
order, and then process_error_block() and get_head_and_birth_txg() can
verify that the dp_config_rwlock is already held.

Additionally, a buffer overrun in `spa_get_errlog()` is corrected.  Many
code paths didn't check if `*count` got to zero, instead continuing to
overwrite past the beginning of the userspace buffer at `uaddr`.

Tested by having some errors in the pool (via `zinject -t data
/path/to/file`), one thread running `zpool iostat 0.001`, and another
thread runs `zfs destroy` (in a loop, although it hits the first time).
This reproduces the problem easily without the fix, and works with the
fix.

Reviewed-by: Mark Maybee <[email protected]>
Reviewed-by: George Amanakis <[email protected]>
Reviewed-by: George Wilson <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Matthew Ahrens <[email protected]>
Closes #14239
Closes #14289 

1 files changed, 1 insertions, 1 deletions

diff --git a/include/sys/spa.h b/include/sys/spa.h
index b260dd328..500eb3491 100644
--- a/include/sys/spa.h
+++ b/include/sys/spa.h
@@ -1146,7 +1146,7 @@ extern nvlist_t *zfs_event_create(spa_t *spa, vdev_t *vd, const char *type,
 extern void zfs_post_remove(spa_t *spa, vdev_t *vd);
 extern void zfs_post_state_change(spa_t *spa, vdev_t *vd, uint64_t laststate);
 extern void zfs_post_autoreplace(spa_t *spa, vdev_t *vd);
-extern uint64_t spa_get_errlog_size(spa_t *spa);
+extern uint64_t spa_approx_errlog_size(spa_t *spa);
 extern int spa_get_errlog(spa_t *spa, void *uaddr, uint64_t *count);
 extern void spa_errlog_rotate(spa_t *spa);
 extern void spa_errlog_drain(spa_t *spa);