[RFC][PATCH 01/27] Add vfsmount writer count

[Dave Hansen <haveblue@us.ibm.com>]

This allows a vfsmount to keep track of how many instances
of files open for write there are at a given time.  This
will be useful if it is ever desired to change a mount 
from r/w to r/o at runtime.

A mount can also potentially refuse to allow writers; this
is why mnt_want_write() has a return value.  However, that
functionality will be added later in the series.  For now,
always allow new writers.

Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
---

 lxc-dave/fs/namespace.c        |    1 +
 lxc-dave/include/linux/mount.h |   14 ++++++++++++++
 2 files changed, 15 insertions(+)

diff -puN fs/namespace.c~convert-permission-to-file-and-vfs fs/namespace.c
--- lxc/fs/namespace.c~convert-permission-to-file-and-vfs	2006-06-07 16:53:11.000000000 -0700
+++ lxc-dave/fs/namespace.c	2006-06-07 16:53:11.000000000 -0700
@@ -66,6 +66,7 @@ struct vfsmount *alloc_vfsmnt(const char
 	if (mnt) {
 		memset(mnt, 0, sizeof(struct vfsmount));
 		atomic_set(&mnt->mnt_count, 1);
+		/* atomic_set(&mnt->writer_count, 0); */
 		INIT_LIST_HEAD(&mnt->mnt_hash);
 		INIT_LIST_HEAD(&mnt->mnt_child);
 		INIT_LIST_HEAD(&mnt->mnt_mounts);
diff -puN include/linux/mount.h~convert-permission-to-file-and-vfs include/linux/mount.h
--- lxc/include/linux/mount.h~convert-permission-to-file-and-vfs	2006-06-07 16:53:11.000000000 -0700
+++ lxc-dave/include/linux/mount.h	2006-06-07 16:53:11.000000000 -0700
@@ -12,6 +12,8 @@
 #define _LINUX_MOUNT_H
 #ifdef __KERNEL__
 
+#include <linux/err.h>
+#include <linux/fs.h>
 #include <linux/types.h>
 #include <linux/list.h>
 #include <linux/spinlock.h>
@@ -38,6 +40,7 @@ struct vfsmount {
 	struct list_head mnt_mounts;	/* list of children, anchored here */
 	struct list_head mnt_child;	/* and going through their mnt_child */
 	atomic_t mnt_count;
+	atomic_t mnt_writers;
 	int mnt_flags;
 	int mnt_expiry_mark;		/* true if marked for expiry */
 	char *mnt_devname;		/* Name of device e.g. /dev/dsk/hda1 */
@@ -58,6 +61,17 @@ static inline struct vfsmount *mntget(st
 	return mnt;
 }
 
+static inline int mnt_want_write(struct vfsmount *mnt)
+{
+	atomic_inc(&mnt->mnt_writers);
+	return 0;
+}
+
+static inline void mnt_drop_write(struct vfsmount *mnt)
+{
+	atomic_dec(&mnt->mnt_writers);
+}
+
 extern void mntput_no_expire(struct vfsmount *mnt);
 extern void mnt_pin(struct vfsmount *mnt);
 extern void mnt_unpin(struct vfsmount *mnt);
diff -L convert-permission-to-file-and-vfs -puN /dev/null /dev/null
diff -L convert-permission-to-file-and-vfs.patch -puN /dev/null /dev/null
_