devfs: save on spurious relocking for devfs_populate

Tested by:	pho

3 files changed, 16 insertions, 2 deletions

diff --git a/sys/fs/devfs/devfs.h b/sys/fs/devfs/devfs.h
index 5f64a2672799..121d6eb6f0b5 100644
--- a/sys/fs/devfs/devfs.h
+++ b/sys/fs/devfs/devfs.h
@@ -192,6 +192,7 @@ char	*devfs_fqpn(char *, struct devfs_mount *, struct devfs_dirent *,
 	    struct componentname *);
 void	devfs_delete(struct devfs_mount *, struct devfs_dirent *, int);
 void	devfs_dirent_free(struct devfs_dirent *);
+bool	devfs_populate_needed(struct devfs_mount *dm);
 void	devfs_populate(struct devfs_mount *);
 void	devfs_cleanup(struct devfs_mount *);
 void	devfs_unmount_final(struct devfs_mount *);
diff --git a/sys/fs/devfs/devfs_devs.c b/sys/fs/devfs/devfs_devs.c
index 417e13e2757a..5df21a3712d0 100644
--- a/sys/fs/devfs/devfs_devs.c
+++ b/sys/fs/devfs/devfs_devs.c
@@ -659,6 +659,13 @@ devfs_populate_loop(struct devfs_mount *dm, int cleanup)
 	return (0);
 }
 
+bool
+devfs_populate_needed(struct devfs_mount *dm)
+{
+
+	return (dm->dm_generation != devfs_generation);
+}
+
 /*
  * The caller needs to hold the dm for the duration of the call.
  */
@@ -668,9 +675,9 @@ devfs_populate(struct devfs_mount *dm)
 	unsigned gen;
 
 	sx_assert(&dm->dm_lock, SX_XLOCKED);
-	gen = devfs_generation;
-	if (dm->dm_generation == gen)
+	if (!devfs_populate_needed(dm))
 		return;
+	gen = devfs_generation;
 	while (devfs_populate_loop(dm, 0))
 		continue;
 	dm->dm_generation = gen;
diff --git a/sys/fs/devfs/devfs_vnops.c b/sys/fs/devfs/devfs_vnops.c
index 5bd12708b0df..f9e29e0b1c74 100644
--- a/sys/fs/devfs/devfs_vnops.c
+++ b/sys/fs/devfs/devfs_vnops.c
@@ -235,6 +235,11 @@ devfs_populate_vp(struct vnode *vp)
 	ASSERT_VOP_LOCKED(vp, "devfs_populate_vp");
 
 	dmp = VFSTODEVFS(vp->v_mount);
+	if (!devfs_populate_needed(dmp)) {
+		sx_xlock(&dmp->dm_lock);
+		goto out_nopopulate;
+	}
+
 	locked = VOP_ISLOCKED(vp);
 
 	sx_xlock(&dmp->dm_lock);
@@ -252,6 +257,7 @@ devfs_populate_vp(struct vnode *vp)
 		devfs_unmount_final(dmp);
 		return (ERESTART);
 	}
+out_nopopulate:
 	if (VN_IS_DOOMED(vp)) {
 		sx_xunlock(&dmp->dm_lock);
 		return (ERESTART);