zfsonlinux/zfs — pull request #1586 by behlendorf (2013-07-11 21:42:53+00:00)
Title: Fix zfsctl_expire_snapshot() deadlock

It is possible for an automounted snapshot which is expiring to
deadlock with a manual unmount of the snapshot. This can occur
because taskq_cancel_id() will block if the task is currently
executing until it completes. But it will never complete because
zfsctl_unmount_snapshot() is holding the zsb->z_ctldir_lock which
zfsctl_expire_snapshot() must acquire.

---------------------- z_unmount/0:2153 ---------------------
mutex_lock z_ctldir_lock>
zfsctl_unmount_snapshot
zfsctl_expire_snapshot
taskq_thread

------------------------- zfs:10690 -------------------------
taskq_wait_id
taskq_cancel_id
__zfsctl_unmount_snapshot
zfsctl_unmount_snapshot z_ctldir_lock>
zfs_unmount_snap
zfs_ioc_destroy_snaps_nvl
zfsdev_ioctl
do_vfs_ioctl

We resolve the deadlock by dropping the zsb->z_ctldir_lock before
calling __zfsctl_unmount_snapshot(). The lock is only there to
prevent concurrent modification to the zsb->z_ctldir_snaps AVL
tree. Moreover, we're careful to remove the zfs_snapentry_t from
the AVL tree before dropping the lock which ensures no other tasks
can find it. On failure it's added back to the tree.

Signed-off-by: Brian Behlendorf
Issue #1527