Re: [PATCH v4] vhost_user: protect active rings from async ring changes

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 20 Dec 2017 16:37:52 +0200
Victor Kaplansky <victork@redhat.com> wrote:

> When performing live migration or memory hot-plugging,
> the changes to the device and vrings made by message handler
> done independently from vring usage by PMD threads.
> 
> This causes for example segfaults during live-migration
> with MQ enable, but in general virtually any request
> sent by qemu changing the state of device can cause
> problems.
> 
> These patches fixes all above issues by adding a spinlock
> to every vring and requiring message handler to start operation
> only after ensuring that all PMD threads related to the device
> are out of critical section accessing the vring data.
> 
> Each vring has its own lock in order to not create contention
> between PMD threads of different vrings and to prevent
> performance degradation by scaling queue pair number.
> 
> See https://bugzilla.redhat.com/show_bug.cgi?id=1450680
> 
> Signed-off-by: Victor Kaplansky <victork@redhat.com>
> ---
> 
> v4:
> 
>  o moved access_unlock before accessing enable flag and
>    access_unlock after iommu_unlock consistently.
>  o cosmetics: removed blank line.
>  o the access_lock variable moved to be in the same
>    cache line with enable and access_ok flags.
>  o dequeue path is now guarded with trylock and returning
>    zero if unsuccessful.
>  o GET_VRING_BASE operation is not guarded by access lock
>    to avoid deadlock with device_destroy. See the comment
>    in the code.
>  o Fixed error path exit from enqueue and dequeue carefully
>    unlocking access and iommu locks as appropriate.
> 
> v3:
>    o Added locking to enqueue flow.
>    o Enqueue path guarded as well as dequeue path.
>    o Changed name of active_lock.
>    o Added initialization of guarding spinlock.
>    o Reworked functions skimming over all virt-queues.
>    o Performance measurements done by Maxime Coquelin shows
>      no degradation in bandwidth and throughput.
>    o Spelling.
>    o Taking lock only on set operations.
>    o IOMMU messages are not guarded by access lock.
> 
> v2:
>    o Fixed checkpatch complains.
>    o Added Signed-off-by.
>    o Refined placement of guard to exclude IOMMU messages.
>    o TODO: performance degradation measurement.
> 
> 
> 
>  lib/librte_vhost/vhost.h      | 25 +++++++++++++--
>  lib/librte_vhost/vhost.c      |  1 +
>  lib/librte_vhost/vhost_user.c | 71 +++++++++++++++++++++++++++++++++++++++++++
>  lib/librte_vhost/virtio_net.c | 28 ++++++++++++++---
>  4 files changed, 119 insertions(+), 6 deletions(-)
> 
> diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h
> index 1cc81c17..f3e43e95 100644
> --- a/lib/librte_vhost/vhost.h
> +++ b/lib/librte_vhost/vhost.h
> @@ -108,12 +108,14 @@ struct vhost_virtqueue {
>  
>  	/* Backend value to determine if device should started/stopped */
>  	int			backend;
> +	int			enabled;
> +	int			access_ok;
> +	rte_spinlock_t		access_lock;

Maybe these int's should be bool?

> +
>  	/* Used to notify the guest (trigger interrupt) */
>  	int			callfd;
>  	/* Currently unused as polling mode is enabled */
>  	int			kickfd;
> -	int			enabled;
> -	int			access_ok;
>  
>  	/* Physical address of used ring, for logging */
>  	uint64_t		log_guest_addr;
> @@ -302,6 +304,25 @@ vhost_log_used_vring(struct virtio_net *dev, struct vhost_virtqueue *vq,
>  	vhost_log_write(dev, vq->log_guest_addr + offset, len);
>  }
>  
> +static __rte_always_inline int
> +vhost_user_access_trylock(struct vhost_virtqueue *vq)
> +{
> +	return rte_spinlock_trylock(&vq->access_lock);
> +}
> +
> +static __rte_always_inline void
> +vhost_user_access_lock(struct vhost_virtqueue *vq)
> +{
> +	rte_spinlock_lock(&vq->access_lock);
> +}
> +
> +static __rte_always_inline void
> +vhost_user_access_unlock(struct vhost_virtqueue *vq)
> +{
> +	rte_spinlock_unlock(&vq->access_lock);
> +}
> +
> +

Wrapping locking inline's adds nothing and makes life harder
for static analysis tools.

The bigger problem is that doing locking on all enqueue/dequeue
can have a visible performance impact. Did you measure that?

Could you invent an RCUish mechanism using compiler barriers?