Re: [PATCH] fs/namespace: don't clobber mnt_hash.next while umounting [v2]

[Linus Torvalds]

On Wed, Mar 19, 2014 at 8:48 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> NAK.  Nice catch, the bug is real, but the fix is wrong.  For one thing,
> you have missed detach_mnt()/attach_mnt(), so you are not covering
> all places where the sucker might be removed from the list.  For another,
> I don't believe that this is the right approach.
>
> The *only* thing we care about is not getting stuck in __lookup_mnt().

Quite frankly, if that's the main issue, then may I suggest aiming to
use a 'hlist' instead of a doubly-linked list? Those have the
advantage that they are NULL-terminated.

Yeah, hlists have some disadvantages too, which might not make them
work in this case, but really, for mnt_hash? hlists are generally
*exactly* what you want for hash lists, because the head is smaller.
And because of the NULL termination rather than having the head used
in the middle of a circular list, you don't get the termination
problems when moving entries across chains.

I did not look whether there was some reason a hlist isn't appropriate
here. Maybe you can tell me. Or go "yeah, a hlist would work nicely".

                Linus
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