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SubjectRe: [patch 3/6] vfs: mountinfo stable peer group id
On Wed, Mar 19, 2008 at 07:37:51PM +0100, Miklos Szeredi wrote:
> set_mnt_shared() is called from namespace.c as well, without
> vfsmount_lock. But agreed, that's not the real issue.

How about the following: let's separate set_mnt_shared() and inventing
group ids. All we need is this:
invent_group_ids(mnt) /* call under namespace_sem */
for all vfsmounts p in subtree rooted at mnt
if p->mnt_share is non-empty
continue
get ID for p
if allocation fails
goto cleanup
return 0
cleanup:
for all vfsmounts q in subtree rooted at mnt
if q == p
break
if q->mnt_share is non-empty
continue
release ID of q
return -ENOMEM

Now here's what we do:
* in do_change_type(), outside of vfsmount_lock, do invent_group_ids()
If it fails - bugger off, if not - proceed as now.
* in attach_recursive_mnt() if IS_MNT_SHARED(dest_mnt) do
invent_group_ids() on the dest_mnt immediately and if it fails do
umount_tree(dest_mnt, 0, ) under vfsmount_lock, then release_mounts()
and bugger off (FWIW, we might want to lift the last part to caller
and do the same to release_mounts() in propagate_mnt()). If it hadn't
failed, we proceed as now.
* in clone_mnt() do
int new_group = group ID of old;
int free_group = 0;
if (flag & (CL_SLAVE | CL_PRIVATE))
new_group = 0; /* not a peer of original */
if ((flag & CL_MAKE_SHARED) && !new_group)
new_group = allocate new ID
if failed
return 0;
free_group = 1;
}
mnt = alloc_vfsmount();
if (mnt) {
set group ID of mnt to new_group;
free_group = 0;
/* as in mainline */
}
if (free_group)
release group ID found in new_group;
return mnt;

then (after allocating new vfsmount) set its group ID to new_group if
alloc_vfsmount() succeeds. Otherwise release group ID if needed and
bugger off as usual.

No need to mess with any additional exclusion for idr protection or with
any kind of retries; allocation failure is allocation failure.

Releasing group ID should be done from do_make_slave(), along with clearing
group ID in vfsmount.

Care to do that using mountinfo-base in vfs-2.6.git as base tree?


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