Re: read_barrier_depends() usage in vhost.c

[Jason Wang <jasowang@redhat.com>]

On 2019/10/19 上午4:58, Will Deacon wrote:
> On Thu, Oct 17, 2019 at 10:17:18AM +0800, Jason Wang wrote:
>> On 2019/10/17 上午7:33, Will Deacon wrote:
>>> In an attempt to remove the remaining traces of [smp_]read_barrier_depends()
>>> following my previous patches to strengthen READ_ONCE() for Alpha [1], I
>>> ended up trying to decipher the read_barrier_depends() usage in the vhost
>>> driver:
>>>
>>> --->8
>>>
>>> // drivers/vhost/vhost.c
>>> static int get_indirect(struct vhost_virtqueue *vq,
>>> 			struct iovec iov[], unsigned int iov_size,
>>> 			unsigned int *out_num, unsigned int *in_num,
>>> 			struct vhost_log *log, unsigned int *log_num,
>>> 			struct vring_desc *indirect)
>>> {
>>> 	[...]
>>>
>>> 	/* We will use the result as an address to read from, so most
>>> 	 * architectures only need a compiler barrier here. */
>>> 	read_barrier_depends();
>>>
>>> --->8
>>>
>>> Unfortunately, although the barrier is commented (hurrah!), it's not
>>> particularly enlightening about the accesses making up the dependency
>>> chain, and I don't understand the supposed need for a compiler barrier
>>> either (read_barrier_depends() doesn't generally provide this).
>>>
>>> Does anybody know which accesses are being ordered here? Usually you'd need
>>> a READ_ONCE()/rcu_dereference() beginning the chain, but I haven't managed
>>> to find one...
>>>
>> I guess it was because we will read from the address stored in the iov like:
>>
>> 1) trasnlate_desc() that stores the userspace buffer pointer in the iov
>>
>> 2) copy_from_iter() that reads from those pointers
> Isn't that exactly the same flow as vhost_copy_from_user(), which doesn't
> have the barrier?


There's a rmb before calling vhost_copy_from_user() (vhost_get_desc()).


>   Staring at the code some more, my best bet at the moment
> is that the load of 'indirect->addr' is probably the one to worry about,
> since it's part of the vring and can be updated concurrently.


I'm also confused about the barrier here, basically in driver side we did:

1) allocate pages

2) store pages in indirect->addr

3) smp_wmb()

4) increase the avail idx (somehow a tail pointer of vring)

in vhost we did:

1) read avail idx

2) smp_rmb()

3) read indirect->addr

4) read from indirect->addr

It looks to me even the data dependency barrier is not necessary since 
we have rmb() which is sufficient for us to the correct indirect->addr 
and driver are not expected to do any writing to indirect->addr after 
avail idx is increased ?

Thanks


>
>> So we need a data dependency barrier in the middle as explained in the
>> memory-barriers.txt? (since READ_ONCE is not used in iov iterator).
> If the barrier is actually required, then there must be a concurrent access
> involved, in which case READ_ONCE should also be used. So I would propose
> something like the diff below, but I'd still be glad to hear whether I'm
> barking up the wrong tree.
>
> Will
>
> --->8
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 36ca2cf419bf..2e370a229fea 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2107,6 +2107,7 @@ static int get_indirect(struct vhost_virtqueue *vq,
>   {
>   	struct vring_desc desc;
>   	unsigned int i = 0, count, found = 0;
> +	__virtio64 addr = READ_ONCE(indirect->addr);
>   	u32 len = vhost32_to_cpu(vq, indirect->len);
>   	struct iov_iter from;
>   	int ret, access;
> @@ -2120,7 +2121,7 @@ static int get_indirect(struct vhost_virtqueue *vq,
>   		return -EINVAL;
>   	}
>   
> -	ret = translate_desc(vq, vhost64_to_cpu(vq, indirect->addr), len, vq->indirect,
> +	ret = translate_desc(vq, vhost64_to_cpu(vq, addr), len, vq->indirect,
>   			     UIO_MAXIOV, VHOST_ACCESS_RO);
>   	if (unlikely(ret < 0)) {
>   		if (ret != -EAGAIN)
> @@ -2129,10 +2130,6 @@ static int get_indirect(struct vhost_virtqueue *vq,
>   	}
>   	iov_iter_init(&from, READ, vq->indirect, ret, len);
>   
> -	/* We will use the result as an address to read from, so most
> -	 * architectures only need a compiler barrier here. */
> -	read_barrier_depends();
> -
>   	count = len / sizeof desc;
>   	/* Buffers are chained via a 16 bit next field, so
>   	 * we can have at most 2^16 of these. */
> @@ -2152,12 +2149,12 @@ static int get_indirect(struct vhost_virtqueue *vq,
>   		}
>   		if (unlikely(!copy_from_iter_full(&desc, sizeof(desc), &from))) {
>   			vq_err(vq, "Failed indirect descriptor: idx %d, %zx\n",
> -			       i, (size_t)vhost64_to_cpu(vq, indirect->addr) + i * sizeof desc);
> +			       i, (size_t)vhost64_to_cpu(vq, addr) + i * sizeof desc);
>   			return -EINVAL;
>   		}
>   		if (unlikely(desc.flags & cpu_to_vhost16(vq, VRING_DESC_F_INDIRECT))) {
>   			vq_err(vq, "Nested indirect descriptor: idx %d, %zx\n",
> -			       i, (size_t)vhost64_to_cpu(vq, indirect->addr) + i * sizeof desc);
> +			       i, (size_t)vhost64_to_cpu(vq, addr) + i * sizeof desc);
>   			return -EINVAL;
>   		}
>   

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