RE: [PATCH v3 1/4] vhost: change vhost_virtqueue access lock to a read/write one

["Xia, Chenbo" <chenbo.xia@intel.com>]

Hi Eelco,

> -----Original Message-----
> From: Eelco Chaudron <echaudro@redhat.com>
> Sent: Wednesday, May 17, 2023 5:09 PM
> To: maxime.coquelin@redhat.com; Xia, Chenbo <chenbo.xia@intel.com>;
> david.marchand@redhat.com
> Cc: dev@dpdk.org
> Subject: [PATCH v3 1/4] vhost: change vhost_virtqueue access lock to a
> read/write one
> 
> This change will allow the vhost interrupt datapath handling to be split
> between two processed without one of them holding an explicit lock.
> 
> Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
> ---
>  lib/eal/include/generic/rte_rwlock.h |   17 ++++++
>  lib/vhost/vhost.c                    |   46 +++++++++--------
>  lib/vhost/vhost.h                    |    4 +-
>  lib/vhost/vhost_user.c               |   14 +++--
>  lib/vhost/virtio_net.c               |   90 +++++++++++++++++------------
> -----
>  5 files changed, 94 insertions(+), 77 deletions(-)
> 
> diff --git a/lib/eal/include/generic/rte_rwlock.h
> b/lib/eal/include/generic/rte_rwlock.h
> index 71e2d8d5f4..9e083bbc61 100644
> --- a/lib/eal/include/generic/rte_rwlock.h
> +++ b/lib/eal/include/generic/rte_rwlock.h
> @@ -236,6 +236,23 @@ rte_rwlock_write_unlock(rte_rwlock_t *rwl)
>  	__atomic_fetch_sub(&rwl->cnt, RTE_RWLOCK_WRITE, __ATOMIC_RELEASE);
>  }
> 
> +/**
> + * Test if the write lock is taken.
> + *
> + * @param rwl
> + *   A pointer to a rwlock structure.
> + * @return
> + *   1 if the write lock is currently taken; 0 otherwise.
> + */
> +static inline int
> +rte_rwlock_write_is_locked(rte_rwlock_t *rwl)
> +{
> +	if (__atomic_load_n(&rwl->cnt, __ATOMIC_RELAXED) & RTE_RWLOCK_WRITE)
> +		return 1;
> +
> +	return 0;
> +}
> +

Again we need to update release note as it's a new EAL API.

>  /**
>   * Try to execute critical section in a hardware memory transaction, if
> it
>   * fails or not available take a read lock
> diff --git a/lib/vhost/vhost.c b/lib/vhost/vhost.c
> index ef37943817..74bdbfd810 100644
> --- a/lib/vhost/vhost.c
> +++ b/lib/vhost/vhost.c
> @@ -393,9 +393,9 @@ free_vq(struct virtio_net *dev, struct vhost_virtqueue
> *vq)
>  	else
>  		rte_free(vq->shadow_used_split);
> 
> -	rte_spinlock_lock(&vq->access_lock);
> +	rte_rwlock_write_lock(&vq->access_lock);
>  	vhost_free_async_mem(vq);
> -	rte_spinlock_unlock(&vq->access_lock);
> +	rte_rwlock_write_unlock(&vq->access_lock);
>  	rte_free(vq->batch_copy_elems);
>  	vhost_user_iotlb_destroy(vq);
>  	rte_free(vq->log_cache);
> @@ -630,7 +630,7 @@ alloc_vring_queue(struct virtio_net *dev, uint32_t
> vring_idx)
> 
>  		dev->virtqueue[i] = vq;
>  		init_vring_queue(dev, vq, i);
> -		rte_spinlock_init(&vq->access_lock);
> +		rte_rwlock_init(&vq->access_lock);
>  		vq->avail_wrap_counter = 1;
>  		vq->used_wrap_counter = 1;
>  		vq->signalled_used_valid = false;
> @@ -1305,14 +1305,14 @@ rte_vhost_vring_call(int vid, uint16_t vring_idx)
>  	if (!vq)
>  		return -1;
> 
> -	rte_spinlock_lock(&vq->access_lock);
> +	rte_rwlock_read_lock(&vq->access_lock);
> 
>  	if (vq_is_packed(dev))
>  		vhost_vring_call_packed(dev, vq);
>  	else
>  		vhost_vring_call_split(dev, vq);
> 
> -	rte_spinlock_unlock(&vq->access_lock);
> +	rte_rwlock_read_unlock(&vq->access_lock);

Not sure about this. vhost_ring_call_packed/split is changing some field in
Vq. Should we use write lock here?

Thanks,
Chenbo