On Mon, Apr 22, 2013 at 11:00:19PM +0800, Asias He wrote:
> On Mon, Apr 22, 2013 at 04:28:07PM +0300, Michael S. Tsirkin wrote:
> > On Mon, Apr 22, 2013 at 04:53:27PM +0800, Asias He wrote:
> > > On Thu, Apr 18, 2013 at 10:38:23AM +0300, Michael S. Tsirkin wrote:
> > > > On Thu, Apr 18, 2013 at 04:32:30PM +0800, Asias He wrote:
> > > > > On Thu, Apr 18, 2013 at 10:09:53AM +0300, Michael S. Tsirkin wrote:
> > > > > > On Thu, Apr 18, 2013 at 09:05:53AM +0800, Asias He wrote:
> > > > > > > Signed-off-by: Asias He <as...@redhat.com>
> > > > > > > ---
> > > > > > > drivers/vhost/tcm_vhost.c | 18 ++++++++++++++++++
> > > > > > > 1 file changed, 18 insertions(+)
> > > > > > >
> > > > > > > diff --git a/drivers/vhost/tcm_vhost.c b/drivers/vhost/tcm_vhost.c
> > > > > > > index 88ebb79..8f05528 100644
> > > > > > > --- a/drivers/vhost/tcm_vhost.c
> > > > > > > +++ b/drivers/vhost/tcm_vhost.c
> > > > > > > @@ -111,6 +111,24 @@ static bool tcm_vhost_check_feature(struct
> > > > > > > vhost_scsi *vs, int feature)
> > > > > > > return ret;
> > > > > > > }
> > > > > > >
> > > > > > > +static bool tcm_vhost_check_endpoint(struct vhost_virtqueue *vq)
> > > > > > > +{
> > > > > > > + bool ret = false;
> > > > > > > +
> > > > > > > + /*
> > > > > > > + * We can handle the vq only after the endpoint is setup by
> > > > > > > calling the
> > > > > > > + * VHOST_SCSI_SET_ENDPOINT ioctl.
> > > > > > > + *
> > > > > > > + * TODO: Check that we are running from vhost_worker which acts
> > > > > > > + * as read-side critical section for vhost kind of RCU.
> > > > > > > + * See the comments in struct vhost_virtqueue in
> > > > > > > drivers/vhost/vhost.h
> > > > > > > + */
> > > > > > > + if (rcu_dereference_check(vq->private_data, 1))
> > > > > > > + ret = true;
> > > > > > > +
> > > > > > > + return ret;
> > > > > > > +}
> > > > > > > +
> > > > > >
> > > > > > You can't use RCU in this way. You need to actually
> > > > > > derefence. Instead, just move this within vq mutex
> > > > > > and do rcu_dereference_protected. document that this function
> > > > > > requires vq mutex.
> > > > >
> > > > > Any difference with:
> > > > >
> > > > > vs_tpg = rcu_dereference_check(vq->private_data, 1);
> > > >
> > > > Yes, it's not the same. rcu_dereference_protected says
> > > > "this is generally RCU but here I use it under lock".
> > > > rcu_dereference_check can only be used if you
> > > > really dereference later, and you don't.
> > >
> > > So you want the helper to be this?
> > >
> > > static bool tcm_vhost_check_endpoint()
> > > {
> > > struct tcm_vhost_tpg **vs_tpg;
> > >
> > > vs_tpg = rcu_dereference_check(vq->private_data, 1);
> > > if (vs_tpg)
> > > return true;
> > > else
> > > return false
> > > }
> > >
> > > And what difference code the compiler will generate with this:
> > >
> > > static bool tcm_vhost_check_endpoint()
> > > {
> > >
> > > if (rcu_dereference_check(vq->private_data, 1))
> > > return true
> > > else
> > > return false
> > > }
> > >
> >
> > No, what I want is that private data is
> > either dereferenced (not tested against NULL)
> > or used under vq mutex.
> > In this case the second option is the easiest
> > and the cleanest. So move it under mutex
> > and then test it safely without rcu.
>
> Why you use it in -net? The test against NULL is under in the rcu
> read critical section. What is wrong with this really?
Look at the code and see that barriers in rcu_dereference
assume that pointer really is dereferenced.
You don't dereference it, so rcu_dereference_XXX is wrong.
> > > >
> > > > > if (vs_tpg)
> > > > > return ture
> > > > > else
> > > > > return false;
> > > > >
> > > > > Note, tcm_vhost_check_endpoint() is called in multiple places. Having
> > > > > a
> > > > > helper is more convenient.
> > > >
> > > > I'm fine with the helper, but fix the implementation
> > > > and fix all callers to have vq mutex when calling.
> > >
> > > All the caller are in the vhost thread. Why need vq mutex to be taken?
> >
> > vhost thread is an implementation detail, people are experimenting with
> > various threading strategies. In particular, code must
> > keep working when we move it to use a workqueue.
>
> Well, you should remove your vhost RCU thing first and use real RCU
> explicitly.
I don't think it's useful or possible.
> Does -net keep working after vhost changes to workqueue?
I think so, yes.
> Since when this becomes a requirement?
It's the requirement to use the abstractions that are in place. Devices
should use the interfaces and not assume vhost implementation, it
happens to be the case but it's not guaranteed.
> > > > > > > static int tcm_vhost_check_true(struct se_portal_group *se_tpg)
> > > > > > > {
> > > > > > > return 1;
> > > > > > > --
> > > > > > > 1.8.1.4
> > > > >
> > > > > --
> > > > > Asias
> > >
> > > --
> > > Asias
>
> --
> Asias
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