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SubjectRe: [PATCH tip/core/rcu 43/48] vhost: add __rcu annotations
On Tue, May 04, 2010 at 04:57:50PM -0700, Paul E. McKenney wrote:
> On Wed, May 05, 2010 at 12:39:36AM +0300, Michael S. Tsirkin wrote:
> > On Tue, May 04, 2010 at 01:19:53PM -0700, Paul E. McKenney wrote:
> > > From: Arnd Bergmann <arnd@relay.de.ibm.com>
> > >
> > > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > Cc: "Michael S. Tsirkin" <mst@redhat.com>
> > > ---
> > > drivers/vhost/net.c | 6 +++---
> > > drivers/vhost/vhost.c | 12 ++++++------
> > > drivers/vhost/vhost.h | 4 ++--
> > > 3 files changed, 11 insertions(+), 11 deletions(-)
> > >
> > > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > > index 9777583..36e8dec 100644
> > > --- a/drivers/vhost/net.c
> > > +++ b/drivers/vhost/net.c
> > > @@ -364,7 +364,7 @@ static void vhost_net_disable_vq(struct vhost_net *n,
> > > static void vhost_net_enable_vq(struct vhost_net *n,
> > > struct vhost_virtqueue *vq)
> > > {
> > > - struct socket *sock = vq->private_data;
> > > + struct socket *sock = rcu_dereference(vq->private_data);
> >
> > This should be rcu_dereference_const as well: it is called
> > with vq mutex held.
>
> How about the following?
>
> struct socket *sock;
>
> sock = rcu_dereference_protected(vq->private_data,
> lockdep_is_held(&vq->mutex));
>
> This could be used for some (though not all) of these situations.
>
> And just so you know... The fact that this is here in the first
> place is actually my mistake -- my intention was to include the __rcu
> annotations and nothing else, then follow up with bug fixes. In fact,
> the alert reader will have noted that there is in fact no such thing
> as rcu_dereference_const(). And have concluded that none of my test
> machines use vhost. :-/
>
> But as long as we are here, might as well complete the annotation...
>
> So I have inserted guesses for the lockdep_is_held() expressions below
> for your amusement. Please let me know what I should be using instead.

I'll go over it. Could you point me to documentation to the API
I should use with this patch?

> > > if (!sock)
> > > return;
> > > if (vq == n->vqs + VHOST_NET_VQ_TX) {
> > > @@ -380,7 +380,7 @@ static struct socket *vhost_net_stop_vq(struct vhost_net *n,
> > > struct socket *sock;
> > >
> > > mutex_lock(&vq->mutex);
> > > - sock = vq->private_data;
> > > + sock = rcu_dereference_const(vq->private_data);
>
> sock = rcu_dereference_protected(vq->private_data,
> lockdep_is_held(&vq->mutex));
>
> > > vhost_net_disable_vq(n, vq);
> > > rcu_assign_pointer(vq->private_data, NULL);
> > > mutex_unlock(&vq->mutex);
> > > @@ -518,7 +518,7 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
> > > }
> > >
> > > /* start polling new socket */
> > > - oldsock = vq->private_data;
> > > + oldsock = rcu_dereference_const(vq->private_data);
>
> oldsock = rcu_dereference_protected(vq->private_data,
> lockdep_is_held(&vq->mutex));
>
> Though I can't say I see where this lock is actually acquired in this
> case...

Just above:

mutex_lock(&vq->mutex);

/* Verify that ring has been setup correctly. */
if (!vhost_vq_access_ok(vq)) {
r = -EFAULT;
goto err_vq;
}
sock = get_socket(fd);
if (IS_ERR(sock)) {
r = PTR_ERR(sock);
goto err_vq;
}

/* start polling new socket */



> > > if (sock == oldsock)
> > > goto done;
> > >
> > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > index e69d238..fc9bde2 100644
> > > --- a/drivers/vhost/vhost.c
> > > +++ b/drivers/vhost/vhost.c
> > > @@ -180,7 +180,7 @@ long vhost_dev_reset_owner(struct vhost_dev *dev)
> > > vhost_dev_cleanup(dev);
> > >
> > > memory->nregions = 0;
> > > - dev->memory = memory;
> > > + rcu_assign_pointer(dev->memory, memory);
> >
> > This is called when there can be no active readers, so the smp_wmb
> > inside rcu_assign_pointer isn't really needed.
> > Use RCU_INIT_POINTER or something like this instead?
>
> Good point! Fixed.
>
> > > return 0;
> > > }
> > >
> > > @@ -212,8 +212,8 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
> > > fput(dev->log_file);
> > > dev->log_file = NULL;
> > > /* No one will access memory at this point */
> > > - kfree(dev->memory);
> > > - dev->memory = NULL;
> > > + kfree(rcu_dereference_const(dev->memory));
>
> kfree(rcu_dereference_protected(dev->memory,
> lockdep_is_held(&dev->mutex));
>
> > > + rcu_assign_pointer(dev->memory, NULL);
> >
> > Same here.
>
> Fixed -- any in any case, we can always use RCU_INIT_POINTER() when
> assigning NULL.
>
> > > if (dev->mm)
> > > mmput(dev->mm);
> > > dev->mm = NULL;
> > > @@ -294,14 +294,14 @@ static int vq_access_ok(unsigned int num,
> > > /* Caller should have device mutex but not vq mutex */
> > > int vhost_log_access_ok(struct vhost_dev *dev)
> > > {
> > > - return memory_access_ok(dev, dev->memory, 1);
> > > + return memory_access_ok(dev, rcu_dereference_const(dev->memory), 1);
>
> return memory_access_ok(dev, rcu_dereference_protected(dev->memory, lockdep_is_held(&dev->mutex)), 1);
>
> And yes, we do need an rcu_dereference_vqdev() wrapper function, but just
> want to identify the mutexes for the moment.
>
> Maybe a separate rcu_dereference_vq() as well -- but you tell me!
>
> > > }
> > >
> > > /* Verify access for write logging. */
> > > /* Caller should have vq mutex and device mutex */
> > > static int vq_log_access_ok(struct vhost_virtqueue *vq, void __user *log_base)
> > > {
> > > - return vq_memory_access_ok(log_base, vq->dev->memory,
> > > + return vq_memory_access_ok(log_base, rcu_dereference(vq->dev->memory),
> >
> > rcu_dereference_const. This is called under vq mutex and the comment
> > above it says as much.
>
> return memory_access_ok(dev, rcu_dereference_protected(dev->memory, lockdep_is_held(&dev->mutex)),
>
> > > vhost_has_feature(vq->dev, VHOST_F_LOG_ALL)) &&
> > > (!vq->log_used || log_access_ok(log_base, vq->log_addr,
> > > sizeof *vq->used +
> > > @@ -342,7 +342,7 @@ static long vhost_set_memory(struct vhost_dev *d, struct vhost_memory __user *m)
> > >
> > > if (!memory_access_ok(d, newmem, vhost_has_feature(d, VHOST_F_LOG_ALL)))
> > > return -EFAULT;
> > > - oldmem = d->memory;
> > > + oldmem = rcu_dereference_const(d->memory);
>
> oldmem = rcu_dereference_protected(d->memory,
> lockdep_is_held(&d->mutex));
>
> > > rcu_assign_pointer(d->memory, newmem);
> > > synchronize_rcu();
> > > kfree(oldmem);
> > > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > > index 44591ba..240396c 100644
> > > --- a/drivers/vhost/vhost.h
> > > +++ b/drivers/vhost/vhost.h
> > > @@ -92,7 +92,7 @@ struct vhost_virtqueue {
> > > * work item execution acts instead of rcu_read_lock() and the end of
> > > * work item execution acts instead of rcu_read_lock().
> > > * Writers use virtqueue mutex. */
> > > - void *private_data;
> > > + void __rcu *private_data;
> > > /* Log write descriptors */
> > > void __user *log_base;
> > > struct vhost_log log[VHOST_NET_MAX_SG];
> > > @@ -102,7 +102,7 @@ struct vhost_dev {
> > > /* Readers use RCU to access memory table pointer
> > > * log base pointer and features.
> > > * Writers use mutex below.*/
> > > - struct vhost_memory *memory;
> > > + struct vhost_memory __rcu *memory;
> > > struct mm_struct *mm;
> > > struct mutex mutex;
> > > unsigned acked_features;
> > > --
> > > 1.7.0


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