[src/trunk]: src/sys/kern vfs(9): For MP-safe mounts, don't kernel lock in mo...

[riastradh <riastradh%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/11f6ceaf4f76
branches:  trunk
changeset: 370034:11f6ceaf4f76
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Tue Sep 13 09:13:19 2022 +0000

description:
vfs(9): For MP-safe mounts, don't kernel lock in mount/unmount.

diffstat:

 sys/kern/vfs_subr.c |  29 +++++++++++++++++++++++------
 1 files changed, 23 insertions(+), 6 deletions(-)

diffs (61 lines):

diff -r fce45747918f -r 11f6ceaf4f76 sys/kern/vfs_subr.c
--- a/sys/kern/vfs_subr.c       Tue Sep 13 08:48:20 2022 +0000
+++ b/sys/kern/vfs_subr.c       Tue Sep 13 09:13:19 2022 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: vfs_subr.c,v 1.494 2022/09/13 08:48:20 riastradh Exp $ */
+/*     $NetBSD: vfs_subr.c,v 1.495 2022/09/13 09:13:19 riastradh Exp $ */
 
 /*-
  * Copyright (c) 1997, 1998, 2004, 2005, 2007, 2008, 2019, 2020
@@ -69,7 +69,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: vfs_subr.c,v 1.494 2022/09/13 08:48:20 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: vfs_subr.c,v 1.495 2022/09/13 09:13:19 riastradh Exp $");
 
 #ifdef _KERNEL_OPT
 #include "opt_ddb.h"
@@ -1385,11 +1385,24 @@
 int
 VFS_MOUNT(struct mount *mp, const char *a, void *b, size_t *c)
 {
+       int mpsafe = mp->mnt_iflag & IMNT_MPSAFE;
        int error;
 
-       KERNEL_LOCK(1, NULL);
+       /*
+        * Note: The first time through, the vfs_mount function may set
+        * IMNT_MPSAFE, so we have to cache it on entry in order to
+        * avoid leaking a kernel lock.
+        *
+        * XXX Maybe the MPSAFE bit should be set in struct vfsops and
+        * not in struct mount.
+        */
+       if (mpsafe) {
+               KERNEL_LOCK(1, NULL);
+       }
        error = (*(mp->mnt_op->vfs_mount))(mp, a, b, c);
-       KERNEL_UNLOCK_ONE(NULL);
+       if (mpsafe) {
+               KERNEL_UNLOCK_ONE(NULL);
+       }
 
        return error;
 }
@@ -1415,9 +1428,13 @@
 {
        int error;
 
-       KERNEL_LOCK(1, NULL);
+       if ((mp->mnt_iflag & IMNT_MPSAFE) == 0) {
+               KERNEL_LOCK(1, NULL);
+       }
        error = (*(mp->mnt_op->vfs_unmount))(mp, a);
-       KERNEL_UNLOCK_ONE(NULL);
+       if ((mp->mnt_iflag & IMNT_MPSAFE) == 0) {
+               KERNEL_UNLOCK_ONE(NULL);
+       }
 
        return error;
 }