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    Subject[PATCH 04/24] r/o bind mounts: stub functions
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    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.

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

    lxc-dave/fs/namespace.c | 46 +++++++++++++++++++++++++++++++++++++++++
    lxc-dave/include/linux/mount.h | 3 ++
    2 files changed, 49 insertions(+)

    diff -puN fs/namespace.c~add-vfsmount-writer-count fs/namespace.c
    --- lxc/fs/namespace.c~add-vfsmount-writer-count 2007-09-17 09:43:57.000000000 -0700
    +++ lxc-dave/fs/namespace.c 2007-09-17 09:43:57.000000000 -0700
    @@ -80,6 +80,52 @@ struct vfsmount *alloc_vfsmnt(const char
    return mnt;
    }

    +/*
    + * Most r/o checks on a fs are for operations that take
    + * discrete amounts of time, like a write() or unlink().
    + * We must keep track of when those operations start
    + * (for permission checks) and when they end, so that
    + * we can determine when writes are able to occur to
    + * a filesystem.
    + */
    +/*
    + * This tells the low-level filesystem that a write is
    + * about to be performed to it, and makes sure that
    + * writes are allowed before returning success. When
    + * the write operation is finished, mnt_drop_write()
    + * must be called. This is effectively a refcount.
    + */
    +int mnt_want_write(struct vfsmount *mnt)
    +{
    + if (__mnt_is_readonly(mnt))
    + return -EROFS;
    + return 0;
    +}
    +EXPORT_SYMBOL_GPL(mnt_want_write);
    +
    +/*
    + * Tells the low-level filesystem that we are done
    + * performing a write to it. Must be matched with
    + * mnt_want_write() call above.
    + */
    +void mnt_drop_write(struct vfsmount *mnt)
    +{
    +}
    +EXPORT_SYMBOL_GPL(mnt_drop_write);
    +
    +/*
    + * This shouldn't be used directly ouside of the VFS.
    + * It does not guarantee that the filesystem will stay
    + * r/w, just that it is right *now*e
    + * mnt_want/drop_write() will _keep_ the filesystem
    + * r/w.
    + */
    +int __mnt_is_readonly(struct vfsmount *mnt)
    +{
    + return (mnt->mnt_sb->s_flags & MS_RDONLY);
    +}
    +EXPORT_SYMBOL_GPL(__mnt_is_readonly);
    +
    int simple_set_mnt(struct vfsmount *mnt, struct super_block *sb)
    {
    mnt->mnt_sb = sb;
    diff -puN include/linux/mount.h~add-vfsmount-writer-count include/linux/mount.h
    --- lxc/include/linux/mount.h~add-vfsmount-writer-count 2007-09-17 09:43:57.000000000 -0700
    +++ lxc-dave/include/linux/mount.h 2007-09-17 09:43:57.000000000 -0700
    @@ -74,9 +74,12 @@ static inline struct vfsmount *mntget(st
    return mnt;
    }

    +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);
    +extern int __mnt_is_readonly(struct vfsmount *mnt);

    static inline void mntput(struct vfsmount *mnt)
    {
    _
    -
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