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

Eric Dumazet
2014-06-03T13:57:43Z

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);
...