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    SubjectRe: [RFC PATCH 5/10] vfs: shared subtree aware bind mounts
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    On Tue, 2005-09-20 at 00:17, Al Viro wrote:
    > On Fri, Sep 16, 2005 at 11:26:19AM -0700, Ram wrote:
    >
    > This patch needs to be split *AND* accompanied by locking rules. It's
    > pretty much the core of the entire thing; if it's possible to offload
    > chunks elsewhere, life would become easier. Locking rules are badly
    > needed, along with the comments re "why can't that mntput()/dput()
    > block under a spinlock", etc.

    Yes will do.

    Also I realized that vfspnode_lock just added more complexity because
    all it protected was already protected by vfsmount_lock. So I am
    cleaning up that lock.



    >
    > BTW, how are you dealing with MS_MOVE?
    In the patch #6 MS_MOVE and pivot_root are handled.
    >
    > > +void do_detach_prepare_mnt(struct vfsmount *mnt)
    > > +{
    > > + mnt->mnt_mountpoint->d_mounted--;
    > > + mntput(mnt->mnt_parent);
    > > + dput(mnt->mnt_mountpoint);
    > > + mnt->mnt_parent = mnt;
    > > +}
    >
    > General note: mntput() should go _after_ dput() when we deal with pairs.
    > Doesn't cost anything, trivially safe.

    ok

    >
    > > if (res) {
    > > spin_lock(&vfsmount_lock);
    > > + clean_propagation_reference(res);
    >
    > Uh-oh... What makes that safe? We do mntput() here; are we guaranteed
    > that these pointers won't be the last references?

    Yes it is safe and it is not releasing the last reference to the mount.
    Will put in a comment there.

    It is releasing a reference to source mount of the bind operation.

    static void inline clean_propagation_reference(struct vfsmount *mnt)
    +{
    + struct vfsmount *p;
    + for (p = mnt; p; p = next_mnt(p, mnt))
    + if (p->mnt_master)
    + mntput(p->mnt_master);
    +}
    +


    > > + spin_lock(&vfspnode_lock);
    > > + propagate_abort_mount(m);
    >
    > Calls do_detach_prepare() -> dput(), mntput(). At the very least such
    > cases need comments...
    >

    ok will add a comment.
    but propagate_abort_mount() is not holding vfsmount_lock,
    it is holding vfspnode_lock. So there should be a problem. But as
    mentioned earlier, even the need for vfspnode_lock is not needed.



    > > +static void __do_make_private(struct vfsmount *mnt)
    > > +{
    > > + __do_make_slave(mnt);
    > > + list_del_init(&mnt->mnt_slave);
    > > + mnt->mnt_master = NULL;
    > > + set_mnt_private(mnt);
    > > +}
    > > +
    > > int do_make_private(struct vfsmount *mnt)
    > > {
    > > /*
    > > * a private mount is nothing but a
    > > * slave mount with no incoming
    > > * propagations.
    > > */
    > > spin_lock(&vfspnode_lock);
    > > - __do_make_slave(mnt);
    > > - list_del_init(&mnt->mnt_slave);
    > > + __do_make_private(mnt);
    > > spin_unlock(&vfspnode_lock);
    > > - mnt->mnt_master = NULL;
    > > - set_mnt_private(mnt);
    > > return 0;
    > > }
    >
    > Why not do that from the very beginning, BTW?

    can be done. will do.

    >
    > > /*
    > > - * a unclonable mount is nothing but a
    > > + * a unclonable mount is a
    > > * private mount which is unclonnable.
    > > */
    > > spin_lock(&vfspnode_lock);
    > > - __do_make_slave(mnt);
    > > - list_del_init(&mnt->mnt_slave);
    > > + __do_make_private(mnt);
    > > spin_unlock(&vfspnode_lock);
    > > - mnt->mnt_master = NULL;
    > > set_mnt_unclonable(mnt);
    > > return 0;
    > > }
    > -
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