Re: [PULL 2/2] vhost: replace rcu with mutex

Michael S. Tsirkin
2014-06-04T19:51:12Z

On Tue, Jun 03, 2014 at 06:57:43AM -0700, Eric Dumazet wrote:
> On Tue, 2014-06-03 at 14:48 +0200, Paolo Bonzini wrote:
> > Il 02/06/2014 23:58, Eric Dumazet ha scritto:
> > > This looks dubious
> > >
> > > What about using kfree_rcu() instead ?
> >
> > It would lead to unbound allocation from userspace.
>
> Look at how we did this in commit
> c3059477fce2d956a0bb3e04357324780c5d8eeb
>
> >
> > > translate_desc() still uses rcu_read_lock(), its not clear if the mutex
> > > is really held.
> >
> > Yes, vhost_get_vq_desc must be called with the vq mutex held.
> >
> > The rcu_read_lock/unlock in translate_desc is unnecessary.
>
> Yep, this is what I pointed out. This is not only necessary, but
> confusing and might be incorrectly copy/pasted in the future.
>
> This patch is a partial one and leaves confusion.
>
> Some places uses the proper
>
> mp = rcu_dereference_protected(dev->memory,
> lockdep_is_held(&dev->mutex));
>
> others use the now incorrect :
>
> rcu_read_lock();
> mp = rcu_dereference(dev->memory);
> ...
>

I agree, working on a cleanup patch on top now.

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MST