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zfsctl_snapdir_lookup should be able to handle an uncovered vnode

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The uncovered vnode is possible because there is no guarantee that
its hold count would go to zero (and it would be inactivated and reclaimed)
immediately after a covering filesystem is unmounted.
So, such a vnode should be expected and it is possible to re-use it
without any trouble.

MFC after:	3 weeks
Sponsored by:	Panzura
This commit is contained in:
Andriy Gapon 2017-09-12 06:06:58 +00:00
parent c09d0da8d1
commit bcab65cab5
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=323483
1 changed files with 25 additions and 10 deletions
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35
sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_ctldir.c
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@ -816,6 +816,12 @@ zfsctl_snapshot_vnode_setup(vnode_t *vp, void *arg)
* Lookup entry point for the 'snapshot' directory. Try to open the
* snapshot if it exist, creating the pseudo filesystem vnode as necessary.
* Perform a mount of the associated dataset on top of the vnode.
* There are four possibilities:
* - the snapshot node and vnode do not exist
* - the snapshot vnode is covered by the mounted snapshot
* - the snapshot vnode is not covered yet, the mount operation is in progress
* - the snapshot vnode is not covered, because the snapshot has been unmounted
* The last two states are transient and should be relatively short-lived.
*/
int
zfsctl_snapdir_lookup(ap)
@ -881,7 +887,7 @@ zfsctl_snapdir_lookup(ap)
/*
* The vnode must be referenced at least by this thread and
* the mounted snapshot or the thread doing the mounting.
* the mount point or the thread doing the mounting.
* There can be more references from concurrent lookups.
*/
KASSERT(vrefcnt(*vpp) > 1, ("found unreferenced mountpoint"));
@ -893,22 +899,31 @@ zfsctl_snapdir_lookup(ap)
if (err != EJUSTRETURN)
return (err);
#ifdef INVARIANTS
/*
* If the vnode not covered yet, then the mount operation
* must be in progress.
* If the vnode is not covered, then either the mount operation
* is in progress or the snapshot has already been unmounted
* but the vnode hasn't been inactivated and reclaimed yet.
* We can try to re-use the vnode in the latter case.
*/
VI_LOCK(*vpp);
KASSERT(((*vpp)->v_iflag & VI_MOUNT) != 0,
("snapshot vnode not covered"));
VI_UNLOCK(*vpp);
#endif
vput(*vpp);
if (((*vpp)->v_iflag & VI_MOUNT) == 0) {
/* Upgrade to exclusive lock in order to:
* - avoid race conditions
* - satisfy the contract of mount_snapshot()
*/
err = VOP_LOCK(*vpp, LK_TRYUPGRADE | LK_INTERLOCK);
if (err == 0)
break;
} else {
VI_UNLOCK(*vpp);
}
/*
* In this situation we can loop on uncontested locks and starve
* In this state we can loop on uncontested locks and starve
* the thread doing the lengthy, non-trivial mount operation.
* So, yield to prevent that from happening.
*/
vput(*vpp);
kern_yield(PRI_USER);
}