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

Roman Penyaev
2019-05-31T15:05:18Z

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