This patch adds two function mnt_want_write() and
mnt_drop_write(). These are used like a lock pair around
and fs operations that might cause a write to the filesystem.
Before these can become useful, we must first cover each
place in the VFS where writes are performed with a
want/drop pair. When that is complete, we can actually
introduce code that will safely check the counts before
allowing r/w<->r/o transitions to occur.
Note that we put the linux/fs.h #include as far down in
mount.h as possible. This is to keep the mount.h->fs.h->
sched.h->mount.h include dependency from biting us.
Signed-off-by: Dave Hansen <haveblue@xxxxxxxxxx>
---
lxc-dave/fs/namespace.c | 13 +++++++++++++
lxc-dave/include/linux/mount.h | 13 +++++++++++++
2 files changed, 26 insertions(+)
diff -puN fs/namespace.c~03-24-add-vfsmount-writer-count fs/namespace.c
--- lxc/fs/namespace.c~03-24-add-vfsmount-writer-count 2007-06-21 23:23:14.000000000 -0700
+++ lxc-dave/fs/namespace.c 2007-06-21 23:23:14.000000000 -0700
@@ -76,6 +76,19 @@ struct vfsmount *alloc_vfsmnt(const char
return mnt;
}
+int mnt_want_write(struct vfsmount *mnt)
+{
+ if (__mnt_is_readonly(mnt))
+ return -EROFS;
+ return 0;
+}
+EXPORT_SYMBOL_GPL(mnt_want_write);
+
+void mnt_drop_write(struct vfsmount *mnt)
+{
+}
+EXPORT_SYMBOL_GPL(mnt_drop_write);
+
int simple_set_mnt(struct vfsmount *mnt, struct super_block *sb)
{
mnt->mnt_sb = sb;
diff -puN include/linux/mount.h~03-24-add-vfsmount-writer-count include/linux/mount.h
--- lxc/include/linux/mount.h~03-24-add-vfsmount-writer-count 2007-06-21 23:23:14.000000000 -0700
+++ lxc-dave/include/linux/mount.h 2007-06-21 23:23:14.000000000 -0700
@@ -70,6 +70,19 @@ static inline struct vfsmount *mntget(st
return mnt;
}
+#include <linux/fs.h>
+
+/*
+ * This shouldn't be used directly ouside of the VFS,
+ * use mnt_want/drop_write() instead.
+ */
+static inline int __mnt_is_readonly(struct vfsmount *mnt)
+{
+ return (mnt->mnt_sb->s_flags & MS_RDONLY);
+}
+
+extern int mnt_want_write(struct vfsmount *mnt);
+extern void mnt_drop_write(struct vfsmount *mnt);
extern void mntput_no_expire(struct vfsmount *mnt);
extern void mnt_pin(struct vfsmount *mnt);
extern void mnt_unpin(struct vfsmount *mnt);
_
-
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