Fix zpl_mount() deadlock
Commit 93b43af10 inadvertently introduced the following scenario which
can result in a deadlock. This issue was most easily reproduced by
LXD containers using a ZFS storage backend but should be reproducible
under any workload which is frequently mounting and unmounting.
- THREAD A --
spa_sync()
spa_sync_upgrades()
rrw_enter(&dp->dp_config_rwlock, RW_WRITER, FTAG); <- Waiting on B- THREAD B --
mount_fs()
zpl_mount()
zpl_mount_impl()
dmu_objset_hold()
dmu_objset_hold_flags()
dsl_pool_hold()
dsl_pool_config_enter()
rrw_enter(&dp->dp_config_rwlock, RW_READER, tag);
sget()
sget_userns()
grab_super()
down_write(&s->s_umount); <- Waiting on C- THREAD C --
cleanup_mnt()
deactivate_super()
down_write(&s->s_umount);
deactivate_locked_super()
zpl_kill_sb()
kill_anon_super()
generic_shutdown_super()
sync_filesystem()
zpl_sync_fs()
zfs_sync()
zil_commit()
txg_wait_synced() <- Waiting on AReviewed by: Alek Pinchuk <apinchuk@datto.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7598
Closes #7659
Closes #7691
Closes #7693