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

Michael S. Tsirkin
2014-06-04T18:12:57Z

On Mon, Jun 02, 2014 at 02:58:00PM -0700, Eric Dumazet wrote:
> On Tue, 2014-06-03 at 00:30 +0300, Michael S. Tsirkin wrote:
> > All memory accesses are done under some VQ mutex.
> > So lock/unlock all VQs is a faster equivalent of synchronize_rcu()
> > for memory access changes.
> > Some guests cause a lot of these changes, so it's helpful
> > to make them faster.
> >
> > Reported-by: "Gonglei (Arei)" <arei.gonglei@huawei.com>
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> > drivers/vhost/vhost.c | 10 +++++++++-
> > 1 file changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 78987e4..1c05e60 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -593,6 +593,7 @@ static long vhost_set_memory(struct vhost_dev *d, struct vhost_memory __user *m)
> > {
> > struct vhost_memory mem, *newmem, *oldmem;
> > unsigned long size = offsetof(struct vhost_memory, regions);
> > + int i;
> >
> > if (copy_from_user(&mem, m, size))
> > return -EFAULT;
> > @@ -619,7 +620,14 @@ static long vhost_set_memory(struct vhost_dev *d, struct vhost_memory __user *m)
> > oldmem = rcu_dereference_protected(d->memory,
> > lockdep_is_held(&d->mutex));
> > rcu_assign_pointer(d->memory, newmem);
> > - synchronize_rcu();
> > +
> > + /* All memory accesses are done under some VQ mutex.
> > + * So below is a faster equivalent of synchronize_rcu()
> > + */
> > + for (i = 0; i < d->nvqs; ++i) {
> > + mutex_lock(&d->vqs[i]->mutex);
> > + mutex_unlock(&d->vqs[i]->mutex);
> > + }
> > kfree(oldmem);
> > return 0;
> > }
>
> This looks dubious
>
> What about using kfree_rcu() instead ?

Unfortunately userspace relies on the fact that no one
uses the old mappings by the time ioctl returns.
The issue isn't freeing the memory.

> translate_desc() still uses rcu_read_lock(), its not clear if the mutex
> is really held.
>

Thanks, good point, we can drop that rcu_read_lock now, but I think this could be a
patch on top.

--
MST