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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 12 Sep 2017, Stefano Stabellini wrote:
> On Tue, 12 Sep 2017, Boris Ostrovsky wrote:
> > 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.
> 
> I thought that READ_ONCE guaranteed atomicity for sizes less or equal to
> the machine word size. It doesn't make any atomicity guarantees for
> sizes >8 bytes.
> 
> 
> > 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.
> 
> I am happy to remove the READ_ONCE and WRITE_ONCE, if we all think it is
> safe.

Looking at other code in Linux, it seems that they are making this
assumption in many places. I'll remove READ/WRITE_ONCE.

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