freebsd-skq/sys/cddl/contrib/opensolaris
Andriy Gapon 4ff1c77d22 zfs: overhaul zfs-vfs glue for vnode life-cycle management
* There is no need for the delayed destruction of znodes via taskqueue,
now that we do not need to fear recursion from getnewvnode into
zfs_inactive and zfs_freebsd_reclaim, thus making znode/vnode state
machine a bit simpler.

* More complete porting of zfs_inactive from Solaris VFS model to FreeBSD
vop_inactive and vop_reclaim model.  All destructive actions are done
in zfs_freebsd_reclaim.
This allows to simplify zfs_zget logic.

* Allow zfs_zget to return a doomed vnode if the current thread already
has an exclusive lock on the vnode.

* Clean up Solaris-isms like bailing out of reclaim/inactive on certain
values of v_usecount (aka v_count) or directly messing with this counter.

* Do not clear z_vnode while znode is still accessible.
z_vnode should be cleared only after zfs_znode_dmu_fini.
Otherwise zfs_zget may get an effectively half-deconstructed znode.
This allows to simplify zfs_zget logic further.

The above changes fix at least two known/reported problems:

o An indefinite wait in the following code path:
vgone -> VOP_RECLAIM -> zfs_freebsd_reclaim -> vnode_destroy_vobject ->
put_pages -> zfs_write -> zil_commit -> zfs_zget
This happened because vgone marks a vnode as VI_DOOMED before calling
VOP_RECLAIM, but zfs_zget would not return a doomed vnode under any
circumstances.
The fix in this change is not complete as it won't fix a deadlock between
two threads doing VOP_RECLAIM where one thread is in zil_commit trying to
zfs_zget a znode/vnode being reclaimed by the other thread, which would be
blocked trying to enter zil_commit.  This type of deadlock has not been
reported as of now.

o An indefinite wait in the unmount path caused by a znode "falling through
the cracks" in inactive+reclaim.  This would happen if the znode is unlinked
while its vnode is still active.

To Do: pass locking flags parameter to zfs_zget, so that the zfs-vfs
glue code doesn't have to re-lock a vnode but could ask for proper locking
from the very start.  This would also allow for the higher level code to
obtain a doomed vnode when it is expected/requested.  Or to avoid blocking
when it is not allowed (see zil_commit example above).

ffs_vgetf seems like a good source of inspiration.

Tested by:	Willem Jan Withagen <wjw@digiware.nl>
MFC after:	6 weeks
2012-11-25 15:33:26 +00:00
..
common Merge recent zfs vendor changes, sync code and adjust userland DEBUG. 2012-09-12 18:05:43 +00:00
uts zfs: overhaul zfs-vfs glue for vnode life-cycle management 2012-11-25 15:33:26 +00:00
OPENSOLARIS.LICENSE