Re: [dpdk-dev] [PATCH v5] vhost_user: protect active rings from async ring changes

["Chen, Junjie J" <junjie.j.chen@intel.com>]

Hi 
May I know why not use trylock also in enqueue path?

Cheers
JJ

> 
> 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>
> ---
> v5:
>  o get rid of spinlock wrapping functions in vhost.h
> 
> 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      |  6 ++--
>  lib/librte_vhost/vhost.c      |  1 +
>  lib/librte_vhost/vhost_user.c | 70
> +++++++++++++++++++++++++++++++++++++++++++
>  lib/librte_vhost/virtio_net.c | 28 ++++++++++++++---
>  4 files changed, 99 insertions(+), 6 deletions(-)
> 
> diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h index
> 1cc81c17..c8f2a817 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;
> +
>  	/* 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;
> diff --git a/lib/librte_vhost/vhost.c b/lib/librte_vhost/vhost.c index
> 4f8b73a0..dcc42fc7 100644
> --- a/lib/librte_vhost/vhost.c
> +++ b/lib/librte_vhost/vhost.c
> @@ -259,6 +259,7 @@ alloc_vring_queue(struct virtio_net *dev, uint32_t
> vring_idx)
> 
>  	dev->virtqueue[vring_idx] = vq;
>  	init_vring_queue(dev, vring_idx);
> +	rte_spinlock_init(&vq->access_lock);
> 
>  	dev->nr_vring += 1;
> 
> diff --git a/lib/librte_vhost/vhost_user.c b/lib/librte_vhost/vhost_user.c
> index f4c7ce46..0685d4e7 100644
> --- a/lib/librte_vhost/vhost_user.c
> +++ b/lib/librte_vhost/vhost_user.c
> @@ -1190,12 +1190,47 @@
> vhost_user_check_and_alloc_queue_pair(struct virtio_net *dev,
> VhostUserMsg *msg)
>  	return alloc_vring_queue(dev, vring_idx);  }
> 
> +static void
> +vhost_user_lock_all_queue_pairs(struct virtio_net *dev) {
> +	unsigned int i = 0;
> +	unsigned int vq_num = 0;
> +
> +	while (vq_num < dev->nr_vring) {
> +		struct vhost_virtqueue *vq = dev->virtqueue[i];
> +
> +		if (vq) {
> +			rte_spinlock_lock(&vq->access_lock);
> +			vq_num++;
> +		}
> +		i++;
> +	}
> +}
> +
> +static void
> +vhost_user_unlock_all_queue_pairs(struct virtio_net *dev) {
> +	unsigned int i = 0;
> +	unsigned int vq_num = 0;
> +
> +	while (vq_num < dev->nr_vring) {
> +		struct vhost_virtqueue *vq = dev->virtqueue[i];
> +
> +		if (vq) {
> +			rte_spinlock_unlock(&vq->access_lock);
> +			vq_num++;
> +		}
> +		i++;
> +	}
> +}
> +
>  int
>  vhost_user_msg_handler(int vid, int fd)  {
>  	struct virtio_net *dev;
>  	struct VhostUserMsg msg;
>  	int ret;
> +	int unlock_required = 0;
> 
>  	dev = get_device(vid);
>  	if (dev == NULL)
> @@ -1241,6 +1276,38 @@ vhost_user_msg_handler(int vid, int fd)
>  		return -1;
>  	}
> 
> +	/*
> +	 * Note: we don't lock all queues on VHOST_USER_GET_VRING_BASE,
> +	 * since it is sent when virtio stops and device is destroyed.
> +	 * destroy_device waits for queues to be inactive, so it is safe.
> +	 * Otherwise taking the access_lock would cause a dead lock.
> +	 */
> +	switch (msg.request.master) {
> +	case VHOST_USER_SET_FEATURES:
> +	case VHOST_USER_SET_PROTOCOL_FEATURES:
> +	case VHOST_USER_SET_OWNER:
> +	case VHOST_USER_RESET_OWNER:
> +	case VHOST_USER_SET_MEM_TABLE:
> +	case VHOST_USER_SET_LOG_BASE:
> +	case VHOST_USER_SET_LOG_FD:
> +	case VHOST_USER_SET_VRING_NUM:
> +	case VHOST_USER_SET_VRING_ADDR:
> +	case VHOST_USER_SET_VRING_BASE:
> +	case VHOST_USER_SET_VRING_KICK:
> +	case VHOST_USER_SET_VRING_CALL:
> +	case VHOST_USER_SET_VRING_ERR:
> +	case VHOST_USER_SET_VRING_ENABLE:
> +	case VHOST_USER_SEND_RARP:
> +	case VHOST_USER_NET_SET_MTU:
> +	case VHOST_USER_SET_SLAVE_REQ_FD:
> +		vhost_user_lock_all_queue_pairs(dev);
> +		unlock_required = 1;
> +		break;
> +	default:
> +		break;
> +
> +	}
> +
>  	switch (msg.request.master) {
>  	case VHOST_USER_GET_FEATURES:
>  		msg.payload.u64 = vhost_user_get_features(dev); @@ -1342,6
> +1409,9 @@ vhost_user_msg_handler(int vid, int fd)
> 
>  	}
> 
> +	if (unlock_required)
> +		vhost_user_unlock_all_queue_pairs(dev);
> +
>  	if (msg.flags & VHOST_USER_NEED_REPLY) {
>  		msg.payload.u64 = !!ret;
>  		msg.size = sizeof(msg.payload.u64);
> diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c index
> 6fee16e5..e09a927d 100644
> --- a/lib/librte_vhost/virtio_net.c
> +++ b/lib/librte_vhost/virtio_net.c
> @@ -44,6 +44,7 @@
>  #include <rte_udp.h>
>  #include <rte_sctp.h>
>  #include <rte_arp.h>
> +#include <rte_spinlock.h>
> 
>  #include "iotlb.h"
>  #include "vhost.h"
> @@ -326,8 +327,11 @@ virtio_dev_rx(struct virtio_net *dev, uint16_t
> queue_id,
>  	}
> 
>  	vq = dev->virtqueue[queue_id];
> +
> +	rte_spinlock_lock(&vq->access_lock);
> +
>  	if (unlikely(vq->enabled == 0))
> -		return 0;
> +		goto out_access_unlock;
> 
>  	if (dev->features & (1ULL << VIRTIO_F_IOMMU_PLATFORM))
>  		vhost_user_iotlb_rd_lock(vq);
> @@ -419,6 +423,9 @@ virtio_dev_rx(struct virtio_net *dev, uint16_t
> queue_id,
>  	if (dev->features & (1ULL << VIRTIO_F_IOMMU_PLATFORM))
>  		vhost_user_iotlb_rd_unlock(vq);
> 
> +out_access_unlock:
> +	rte_spinlock_unlock(&vq->access_lock);
> +
>  	return count;
>  }
> 
> @@ -651,8 +658,11 @@ virtio_dev_merge_rx(struct virtio_net *dev,
> uint16_t queue_id,
>  	}
> 
>  	vq = dev->virtqueue[queue_id];
> +
> +	rte_spinlock_lock(&vq->access_lock);
> +
>  	if (unlikely(vq->enabled == 0))
> -		return 0;
> +		goto out_access_unlock;
> 
>  	if (dev->features & (1ULL << VIRTIO_F_IOMMU_PLATFORM))
>  		vhost_user_iotlb_rd_lock(vq);
> @@ -715,6 +725,9 @@ virtio_dev_merge_rx(struct virtio_net *dev, uint16_t
> queue_id,
>  	if (dev->features & (1ULL << VIRTIO_F_IOMMU_PLATFORM))
>  		vhost_user_iotlb_rd_unlock(vq);
> 
> +out_access_unlock:
> +	rte_spinlock_unlock(&vq->access_lock);
> +
>  	return pkt_idx;
>  }
> 
> @@ -1180,9 +1193,13 @@ rte_vhost_dequeue_burst(int vid, uint16_t
> queue_id,
>  	}
> 
>  	vq = dev->virtqueue[queue_id];
> -	if (unlikely(vq->enabled == 0))
> +
> +	if (unlikely(rte_spinlock_trylock(&vq->access_lock) == 0))
>  		return 0;
> 
> +	if (unlikely(vq->enabled == 0))
> +		goto out_access_unlock;
> +
>  	vq->batch_copy_nb_elems = 0;
> 
>  	if (dev->features & (1ULL << VIRTIO_F_IOMMU_PLATFORM)) @@
> -1240,7 +1257,7 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
>  		if (rarp_mbuf == NULL) {
>  			RTE_LOG(ERR, VHOST_DATA,
>  				"Failed to allocate memory for mbuf.\n");
> -			return 0;
> +			goto out;
>  		}
> 
>  		if (make_rarp_packet(rarp_mbuf, &dev->mac)) { @@ -1356,6
> +1373,9 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
>  	if (dev->features & (1ULL << VIRTIO_F_IOMMU_PLATFORM))
>  		vhost_user_iotlb_rd_unlock(vq);
> 
> +out_access_unlock:
> +	rte_spinlock_unlock(&vq->access_lock);
> +
>  	if (unlikely(rarp_mbuf != NULL)) {
>  		/*
>  		 * Inject it to the head of "pkts" array, so that switch's mac
> --
> Victor