netdev - Re: [PATCH net-next v6 1/4] net: vhost: lock the vqs one by one

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Date:   Mon, 30 Jul 2018 10:54:28 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Tonghao Zhang <xiangxia.m.yue@...il.com>, mst@...hat.com
Cc:     makita.toshiaki@....ntt.co.jp,
        virtualization@...ts.linux-foundation.org,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v6 1/4] net: vhost: lock the vqs one by one



On 2018年07月25日 20:05, Tonghao Zhang wrote:
> On Sun, Jul 22, 2018 at 11:26 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>> On Sat, Jul 21, 2018 at 11:03:59AM -0700, xiangxia.m.yue@...il.com wrote:
>>> From: Tonghao Zhang <xiangxia.m.yue@...il.com>
>>>
>>> This patch changes the way that lock all vqs
>>> at the same, to lock them one by one. It will
>>> be used for next patch to avoid the deadlock.
>>>
>>> Signed-off-by: Tonghao Zhang <xiangxia.m.yue@...il.com>
>>> Acked-by: Jason Wang <jasowang@...hat.com>
>>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>>> ---
>>>   drivers/vhost/vhost.c | 24 +++++++-----------------
>>>   1 file changed, 7 insertions(+), 17 deletions(-)
>>>
>>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>>> index a502f1a..a1c06e7 100644
>>> --- a/drivers/vhost/vhost.c
>>> +++ b/drivers/vhost/vhost.c
>>> @@ -294,8 +294,11 @@ static void vhost_vq_meta_reset(struct vhost_dev *d)
>>>   {
>>>        int i;
>>>
>>> -     for (i = 0; i < d->nvqs; ++i)
>>> +     for (i = 0; i < d->nvqs; ++i) {
>>> +             mutex_lock(&d->vqs[i]->mutex);
>>>                __vhost_vq_meta_reset(d->vqs[i]);
>>> +             mutex_unlock(&d->vqs[i]->mutex);
>>> +     }
>>>   }
>>>
>>>   static void vhost_vq_reset(struct vhost_dev *dev,
>>> @@ -890,20 +893,6 @@ static inline void __user *__vhost_get_user(struct vhost_virtqueue *vq,
>>>   #define vhost_get_used(vq, x, ptr) \
>>>        vhost_get_user(vq, x, ptr, VHOST_ADDR_USED)
>>>
>>> -static void vhost_dev_lock_vqs(struct vhost_dev *d)
>>> -{
>>> -     int i = 0;
>>> -     for (i = 0; i < d->nvqs; ++i)
>>> -             mutex_lock_nested(&d->vqs[i]->mutex, i);
>>> -}
>>> -
>>> -static void vhost_dev_unlock_vqs(struct vhost_dev *d)
>>> -{
>>> -     int i = 0;
>>> -     for (i = 0; i < d->nvqs; ++i)
>>> -             mutex_unlock(&d->vqs[i]->mutex);
>>> -}
>>> -
>>>   static int vhost_new_umem_range(struct vhost_umem *umem,
>>>                                u64 start, u64 size, u64 end,
>>>                                u64 userspace_addr, int perm)
>>> @@ -953,7 +942,10 @@ static void vhost_iotlb_notify_vq(struct vhost_dev *d,
>>>                if (msg->iova <= vq_msg->iova &&
>>>                    msg->iova + msg->size - 1 > vq_msg->iova &&
>>>                    vq_msg->type == VHOST_IOTLB_MISS) {
>>> +                     mutex_lock(&node->vq->mutex);
>>>                        vhost_poll_queue(&node->vq->poll);
>>> +                     mutex_unlock(&node->vq->mutex);
>>> +
>>>                        list_del(&node->node);
>>>                        kfree(node);
>>>                }
>>> @@ -985,7 +977,6 @@ static int vhost_process_iotlb_msg(struct vhost_dev *dev,
>>>        int ret = 0;
>>>
>>>        mutex_lock(&dev->mutex);
>>> -     vhost_dev_lock_vqs(dev);
>>>        switch (msg->type) {
>>>        case VHOST_IOTLB_UPDATE:
>>>                if (!dev->iotlb) {
>>> @@ -1019,7 +1010,6 @@ static int vhost_process_iotlb_msg(struct vhost_dev *dev,
>>>                break;
>>>        }
>>>
>>> -     vhost_dev_unlock_vqs(dev);
>>>        mutex_unlock(&dev->mutex);
>>>
>>>        return ret;
>> I do prefer the finer-grained locking but I remember we
>> discussed something like this in the past and Jason saw issues
>> with such a locking.
> This change is suggested by Jason. Should I send new version because
> the patch 3 is changed.
>
>> Jason?

Actually, the code was a little bit tricky here. Since it assumes 
handle_tx() and handle_rx() run on a single thread. Though the lock 
ordering is different, it was still safe.

Maybe we can add some comments to explain this.

Thanks

>>
>>> --
>>> 1.8.3.1