Re: [PATCH v3 07/13] epoll: call ep_add_event_to_uring() from ep_poll_callback()

Peter Zijlstra
2019-05-31T13:05:28Z

On Fri, May 31, 2019 at 01:22:54PM +0200, Roman Penyaev wrote:
> On 2019-05-31 11:56, Peter Zijlstra wrote:
> > On Thu, May 16, 2019 at 10:58:04AM +0200, Roman Penyaev wrote:

> > > +static inline bool ep_clear_public_event_bits(struct epitem *epi)
> > > +{
> > > + __poll_t old, flags;
> > > +
> > > + /*
> > > + * Here we race with ourselves and with ep_modify(), which can
> > > + * change the event bits. In order not to override events updated
> > > + * by ep_modify() we have to do cmpxchg.
> > > + */
> > > +
> > > + old = epi->event.events;
> > > + do {
> > > + flags = old;
> > > + } while ((old = cmpxchg(&epi->event.events, flags,
> > > + flags & EP_PRIVATE_BITS)) != flags);
> > > +
> > > + return flags & ~EP_PRIVATE_BITS;
> > > +}
> >
> > AFAICT epi->event.events also has normal writes to it, eg. in
> > ep_modify(). A number of architectures cannot handle concurrent normal
> > writes and cmpxchg() to the same variable.
>
> Yes, we race with the current function and with ep_modify(). Then,
> ep_modify()
> should do something as the following:
>
> - epi->event.events = event->events
> + xchg(&epi->event.events, event->events);
>
> Is that ok?

That should be correct, but at that point I think we should also always
read the thing with READ_ONCE() to avoid load-tearing. And I suspect it
then becomes sensible to change the type to atomic_t.

atomic_set() vs atomic_cmpxchg() only carries the extra overhead on
those 'dodgy' platforms.

> Just curious: what are these archs?

Oh, lovely stuff like parisc, sparc32 and arc-eznps. See
arch/parisc/lib/bitops.c:__cmpxchg_*() for example :/ Those systems only
have a single truly atomic op (something from the xchg / test-and-set
family) and the rest is fudged on top of that.