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

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 5/31/23 08:37, Xia, Chenbo wrote:
> 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?

I don't think so, the purpose of the access_lock is not to make the
datapath threads-safe, but to protect the datapath from metadata changes
by the control path.

Thanks,
Maxime
> 
> Thanks,
> Chenbo
>