Re: [PATCH tip/core/rcu 43/48] vhost: add __rcu annotations

[Paul E. McKenney]

On Wed, May 05, 2010 at 02:59:56AM +0300, Michael S. Tsirkin wrote:
> 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@xxxxxxxxxxxxxxxx>
> > > > 
> > > > Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> > > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > > > Cc: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
> > > > ---
> > > >  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?

Documentation/RCU/whatisRCU.txt in current mainline lists the APIs.
This patchset, especially 31/48, gets the in-kernel docbook into
shape.

> > > >  	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 */

Ah!  Color me blind, as usual!  ;-)

							Thanx, Paul

> > > >  	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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