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

[Roman Penyaev]

On 2019-05-31 11:56, Peter Zijlstra wrote:
> On Thu, May 16, 2019 at 10:58:04AM +0200, Roman Penyaev wrote:
> > Each ep_poll_callback() is called when fd calls wakeup() on epfd.
> > So account new event in user ring.
> >
> > The tricky part here is EPOLLONESHOT.  Since we are lockless we
> > have to be deal with ep_poll_callbacks() called in paralle, thus
> > use cmpxchg to clear public event bits and filter out concurrent
> > call from another cpu.
> >
> > Signed-off-by: Roman Penyaev <rpenyaev@xxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> > Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> >
> > diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> > index 2f551c005640..55612da9651e 100644
> > --- a/fs/eventpoll.c
> > +++ b/fs/eventpoll.c
> > @@ -1407,6 +1407,29 @@ struct file *get_epoll_tfile_raw_ptr(struct 
> > file *file, int tfd,
> >  }
> >  #endif /* CONFIG_CHECKPOINT_RESTORE */
> >
> > +/**
> > + * Atomically clear public event bits and return %true if the old 
> > value has
> > + * public event bits set.
> > + */
> > +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?

Just curious: what are these archs?

Thanks.

--
Roman