Changed spa_export_common() such that it no longer holds the
spa_namespace_lock for the entire duration and instead sets
spa_export_thread to indicate an import is in progress on the
spa. This allows for an export to a diffent pool to proceed
in parallel while an export is still processing potentially
long operations like spa_unload_log_sm_flush_all().
Calls like spa_lookup() and spa_vdev_enter() that rely on
the spa_namespace_lock to serialize them against a concurrent
export, now wait for any in-progress export thread to complete
before proceeding.
The 'zpool import -a' sub-command also provides multi-threaded
support, using a thread pool to submit the exports in parallel.
Sponsored-By: Klara Inc.
Sponsored-by: Wasabi Technology, Inc.
Signed-off-by: Don Brady <don.brady@klarasystems.com>
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