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

[Roman Penyaev]

On 2019-05-31 15:05, Peter Zijlstra wrote:
> 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.

But it seems if we afraid of load tearing that should be fixed 
separately,
independently of this patchset, because epi->event.events is updated
in ep_modify() and races with ep_poll_callback(), which reads the value
in couple of places.

Probably nothing terrible will happen, because eventually event comes
or just ignored.


> 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.

Locks, nice.

--
Roman