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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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