git: d0bb255d1fcb - stable/14 - VFS: update VOP_FSYNC() debug check to reflect actual locking policy

The branch stable/14 has been updated by jah:

URL: https://cgit.FreeBSD.org/src/commit/?id=d0bb255d1fcbf4baff4cc43df684deb2ecd2f96d

commit d0bb255d1fcbf4baff4cc43df684deb2ecd2f96d
Author:     Jason A. Harmening <jah@FreeBSD.org>
AuthorDate: 2023-12-26 02:02:25 +0000
Commit:     Jason A. Harmening <jah@FreeBSD.org>
CommitDate: 2024-03-04 18:30:27 +0000

    VFS: update VOP_FSYNC() debug check to reflect actual locking policy
    
    Shared vs. exclusive locking is determined not by MNT_EXTENDED_SHARED
    but by MNT_SHARED_WRITES (although there are several places that
    ignore this and simply always use an exclusive lock).  Also add a
    comment on the possible difference between VOP_GETWRITEMOUNT(vp)
    and vp->v_mount on this path.
    
    Found by local testing of unionfs atop ZFS with DEBUG_VFS_LOCKS.
    
    Reviewed by:    kib, olce
    Differential Revision: https://reviews.freebsd.org/D43816
    
    (cherry picked from commit 9530182e371dee382b76d8594f65633a304b396f)
---
 sys/kern/vfs_subr.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/sys/kern/vfs_subr.c b/sys/kern/vfs_subr.c
index b9e92b5b9812..d1c17dca37d4 100644
--- a/sys/kern/vfs_subr.c
+++ b/sys/kern/vfs_subr.c
@@ -5845,7 +5845,22 @@ vop_fsync_debugprepost(struct vnode *vp, const char *name)
 {
 	if (vp->v_type == VCHR)
 		;
-	else if (MNT_EXTENDED_SHARED(vp->v_mount))
+	/*
+	 * The shared vs. exclusive locking policy for fsync()
+	 * is actually determined by vp's write mount as indicated
+	 * by VOP_GETWRITEMOUNT(), which for stacked filesystems
+	 * may not be the same as vp->v_mount.  However, if the
+	 * underlying filesystem which really handles the fsync()
+	 * supports shared locking, the stacked filesystem must also
+	 * be prepared for its VOP_FSYNC() operation to be called
+	 * with only a shared lock.  On the other hand, if the
+	 * stacked filesystem claims support for shared write
+	 * locking but the underlying filesystem does not, and the
+	 * caller incorrectly uses a shared lock, this condition
+	 * should still be caught when the stacked filesystem
+	 * invokes VOP_FSYNC() on the underlying filesystem.
+	 */
+	else if (MNT_SHARED_WRITES(vp->v_mount))
 		ASSERT_VOP_LOCKED(vp, name);
 	else
 		ASSERT_VOP_ELOCKED(vp, name);