Re: [PATCH v3 11/13] xen/pvcalls: implement poll command

[Boris Ostrovsky]

On 09/12/2017 06:17 PM, Stefano Stabellini wrote:
> On Tue, 12 Sep 2017, Boris Ostrovsky wrote:
>>>>> +
>>>>> +unsigned int pvcalls_front_poll(struct file *file, struct socket *sock,
>>>>> +			       poll_table *wait)
>>>>> +{
>>>>> +	struct pvcalls_bedata *bedata;
>>>>> +	struct sock_mapping *map;
>>>>> +
>>>>> +	if (!pvcalls_front_dev)
>>>>> +		return POLLNVAL;
>>>>> +	bedata = dev_get_drvdata(&pvcalls_front_dev->dev);
>>>>> +
>>>>> +	map = (struct sock_mapping *) READ_ONCE(sock->sk->sk_send_head);
>>>> I just noticed this --- why is it READ_ONCE? Are you concerned that
>>>> sk_send_head may change?
>>> No, but I wanted to avoid partial reads. A caller could call
>>> pvcalls_front_accept and pvcalls_front_poll on newsock almost at the
>>> same time (it is probably not the correct way to use the API), I wanted
>>> to make sure that "map" is either read correctly, or not read at all.
>> How can you have a partial read on a pointer?
> I don't think that the compiler makes any promises on translating a
> pointer read into a single read instruction. Of couse, I expect gcc to
> actually do it without any need for READ/WRITE_ONCE.

READ_ONCE() only guarantees ordering but not atomicity. It resolves (for
64-bit pointers) to

        case 8: *(__u64 *)res = *(volatile __u64 *)p; break;

so if compiler was breaking accesses into two then nothing would have
prevented it from breaking them here (I don't think volatile declaration
would affect this). Moreover, for sizes >8 bytes  READ_ONCE() is
__builtin_memcpy() which is definitely not atomic.

So you can't rely on READ_ONCE being atomic from that perspective.

OTOH, I am pretty sure pointer accesses are guaranteed to be atomic. For
example, atomic64_read() is READ_ONCE(u64), which (per above) is
dereferencing of a 64-bit pointer in C.

-boris