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unionfs: upgrade the vnode lock during fsync() if necessary

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If the underlying upper FS supports shared locking for write ops,
as is the case with ZFS, VOP_FSYNC() may only be called with the vnode
lock held shared.  In this case, temporarily upgrade the lock for
those unionfs maintenance operations which require exclusive locking.

While here, make unionfs inherit the upper FS' support for shared
write locking.  Since the upper FS is the target of VOP_GETWRITEMOUNT()
this is what will dictate the locking behavior of any unionfs caller
that uses vn_start_write() + vn_lktype_write(), so unionfs must be
prepared for the caller to only hold a shared vnode lock in these
cases.

Found in local testing of unionfs atop ZFS with DEBUG_VFS_LOCKS.

MFC after:	2 weeks
Reviewed by:	kib, olce
Differential Revision: https://reviews.freebsd.org/D43817
This commit is contained in:
Jason A. Harmening 2023-12-23 22:42:28 -06:00
parent 9530182e37
commit 2656fc29be
2 changed files with 10 additions and 1 deletions
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3
sys/fs/unionfs/union_vfsops.c
View file

@ -348,7 +348,8 @@ unionfs_domount(struct mount *mp)
if ((ump->um_lowermp->mnt_flag & MNT_LOCAL) != 0 &&
(ump->um_uppermp->mnt_flag & MNT_LOCAL) != 0)
mp->mnt_flag |= MNT_LOCAL;
mp->mnt_kern_flag |= MNTK_NOMSYNC | MNTK_UNIONFS;
mp->mnt_kern_flag |= MNTK_NOMSYNC | MNTK_UNIONFS |
(ump->um_uppermp->mnt_kern_flag & MNTK_SHARED_WRITES);
MNT_IUNLOCK(mp);
/*

8
sys/fs/unionfs/union_vnops.c
View file

@ -998,14 +998,22 @@ unionfs_fsync(struct vop_fsync_args *ap)
struct unionfs_node *unp;
struct unionfs_node_status *unsp;
struct vnode *ovp;
enum unionfs_lkupgrade lkstatus;
KASSERT_UNIONFS_VNODE(ap->a_vp);
unp = VTOUNIONFS(ap->a_vp);
lkstatus = unionfs_upgrade_lock(ap->a_vp);
if (lkstatus == UNIONFS_LKUPGRADE_DOOMED) {
unionfs_downgrade_lock(ap->a_vp, lkstatus);
return (ENOENT);
}
unionfs_get_node_status(unp, ap->a_td, &unsp);
ovp = (unsp->uns_upper_opencnt ? unp->un_uppervp : unp->un_lowervp);
unionfs_tryrem_node_status(unp, unsp);
unionfs_downgrade_lock(ap->a_vp, lkstatus);
if (ovp == NULLVP)
return (EBADF);