nullfs: Slightly reduce contention by reducing concurrent sections

In null_lock_prep_with_smr(), initialize 'lvp' outside of the
SMR-protected section.

In null_lock(), if after locking the lower vnode we notice that we have
been reclaimed, we have to unlock the lower vnode and then relock our
own now that the lock isn't shared anymore.  Call VOP_UNLOCK() on the
lower vnode as soon as this condition is known.

This applies comments from D38761, one of which was missed and the other
added too late.

Reviewed by:    kib
MFC with:       641a58239520 ("nullfs: avoid the interlock in null_lock with smr")
Sponsored by:   The FreeBSD Foundation
Differential Revision:  https://reviews.freebsd.org/D52935

1 files changed, 4 insertions, 3 deletions

diff --git a/sys/fs/nullfs/null_vnops.c b/sys/fs/nullfs/null_vnops.c
index 375b6aa27531..ec8a6b10b13f 100644
--- a/sys/fs/nullfs/null_vnops.c
+++ b/sys/fs/nullfs/null_vnops.c
@@ -788,10 +788,10 @@ null_lock_prep_with_smr(struct vop_lock1_args *ap)
 	struct null_node *nn;
 	struct vnode *lvp;
 
-	vfs_smr_enter();
-
 	lvp = NULL;
 
+	vfs_smr_enter();
+
 	nn = VTONULL_SMR(ap->a_vp);
 	if (__predict_true(nn != NULL)) {
 		lvp = nn->null_lowervp;
@@ -855,6 +855,8 @@ null_lock(struct vop_lock1_args *ap)
 	 * case by reacquiring correct lock in requested mode.
 	 */
 	if (VTONULL(ap->a_vp) == NULL && error == 0) {
+		VOP_UNLOCK(lvp);
+
 		flags = ap->a_flags;
 		ap->a_flags &= ~LK_TYPE_MASK;
 		switch (flags & LK_TYPE_MASK) {
@@ -869,7 +871,6 @@ null_lock(struct vop_lock1_args *ap)
 			panic("Unsupported lock request %d\n",
 			    flags);
 		}
-		VOP_UNLOCK(lvp);
 		error = vop_stdlock(ap);
 	}
 	vdrop(lvp);