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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 <blocking on zsb->z_ctldir_lock> zfsctl_unmount_snapshot zfsctl_expire_snapshot taskq_thread ------------------------- zfs:10690 ------------------------- taskq_wait_id <waiting for z_unmount to exit> taskq_cancel_id __zfsctl_unmount_snapshot zfsctl_unmount_snapshot <takes zsb->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 <behlendorf1@llnl.gov> Signed-off-by: Chris Dunlap <cdunlap@llnl.gov> Closes #1527 |
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Native ZFS for Linux!
ZFS is an advanced file system and volume manager which was originally developed for Solaris and is now maintained by the Illumos community.
ZFS on Linux, which is also known as ZoL, is currently feature complete. It includes fully functional and stable SPA, DMU, ZVOL, and ZPL layers.
Full documentation for installing ZoL on your favorite Linux distribution can be found at: http://zfsonlinux.org