[PATCH v3 1/4] vhost: change vhost_virtqueue access lock to a read/write one
Maxime Coquelin
maxime.coquelin at redhat.com
Wed May 31 11:27:31 CEST 2023
On 5/31/23 08:37, Xia, Chenbo wrote:
> Hi Eelco,
>
>> -----Original Message-----
>> From: Eelco Chaudron <echaudro at redhat.com>
>> Sent: Wednesday, May 17, 2023 5:09 PM
>> To: maxime.coquelin at redhat.com; Xia, Chenbo <chenbo.xia at intel.com>;
>> david.marchand at redhat.com
>> Cc: dev at 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 at 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
>
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