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Date: Tue, 4 May 2010 17:39:39 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
dipankar@...ibm.com, akpm@...ux-foundation.org,
mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de,
peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
dhowells@...hat.com, eric.dumazet@...il.com,
Arnd Bergmann <arnd@...ay.de.ibm.com>,
Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH tip/core/rcu 43/48] vhost: add __rcu annotations
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@...ay.de.ibm.com>
> > > >
> > > > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> > > > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > > > Cc: "Michael S. Tsirkin" <mst@...hat.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?
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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