[src/trunk]: src/sys Add a small comment on buffer cache locking, fix mark le...

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

details:   https://anonhg.NetBSD.org/src/rev/6b67bb2c98df
branches:  trunk
changeset: 748937:6b67bb2c98df
user:      rmind <rmind%NetBSD.org@localhost>
date:      Wed Nov 11 09:15:42 2009 +0000

description:
Add a small comment on buffer cache locking, fix mark letter b_objlock.

diffstat:

 sys/kern/vfs_bio.c |  20 ++++++++++++++++++--
 sys/sys/buf.h      |   4 ++--
 2 files changed, 20 insertions(+), 4 deletions(-)

diffs (59 lines):

diff -r 44da05ceebb0 -r 6b67bb2c98df sys/kern/vfs_bio.c
--- a/sys/kern/vfs_bio.c        Wed Nov 11 09:11:26 2009 +0000
+++ b/sys/kern/vfs_bio.c        Wed Nov 11 09:15:42 2009 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: vfs_bio.c,v 1.220 2009/11/11 07:22:33 rmind Exp $      */
+/*     $NetBSD: vfs_bio.c,v 1.221 2009/11/11 09:15:42 rmind Exp $      */
 
 /*-
  * Copyright (c) 2007, 2008, 2009 The NetBSD Foundation, Inc.
@@ -100,14 +100,30 @@
  */
 
 /*
+ * The buffer cache subsystem.
+ *
  * Some references:
  *     Bach: The Design of the UNIX Operating System (Prentice Hall, 1986)
  *     Leffler, et al.: The Design and Implementation of the 4.3BSD
  *             UNIX Operating System (Addison Welley, 1989)
+ *
+ * Locking
+ *
+ * There are three locks:
+ * - bufcache_lock: protects global buffer cache state.
+ * - BC_BUSY: a long term per-buffer lock.
+ * - buf_t::b_objlock: lock on completion (biowait vs biodone).
+ *
+ * For buffers associated with vnodes (a most common case) b_objlock points
+ * to the vnode_t::v_interlock.  Otherwise, it points to generic buffer_lock.
+ *
+ * Lock order:
+ *     bufcache_lock ->
+ *             buf_t::b_objlock
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: vfs_bio.c,v 1.220 2009/11/11 07:22:33 rmind Exp $");
+__KERNEL_RCSID(0, "$NetBSD: vfs_bio.c,v 1.221 2009/11/11 09:15:42 rmind Exp $");
 
 #include "fs_ffs.h"
 #include "opt_bufcache.h"
diff -r 44da05ceebb0 -r 6b67bb2c98df sys/sys/buf.h
--- a/sys/sys/buf.h     Wed Nov 11 09:11:26 2009 +0000
+++ b/sys/sys/buf.h     Wed Nov 11 09:15:42 2009 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: buf.h,v 1.114 2009/11/11 07:22:34 rmind Exp $ */
+/*     $NetBSD: buf.h,v 1.115 2009/11/11 09:15:42 rmind Exp $ */
 
 /*-
  * Copyright (c) 1999, 2000, 2007, 2008 The NetBSD Foundation, Inc.
@@ -95,7 +95,7 @@
  * b   thread of execution that holds BC_BUSY, does not correspond
  *       directly to any particular LWP
  * c   bufcache_lock
- * l   b_objlock
+ * o   b_objlock
  *
  * For buffers associated with a vnode, b_objlock points to vp->v_interlock.
  * If not associated with a vnode, it points to the generic buffer_lock.