Re: [PATCH v3 00/13] epoll: support pollable epoll from userspace

[Roman Penyaev]

On 2019-05-31 18:33, Peter Zijlstra wrote:
> On Thu, May 16, 2019 at 10:57:57AM +0200, Roman Penyaev wrote:
> > When new event comes for some epoll item kernel does the following:
> >
> >  struct epoll_uitem *uitem;
> >
> >  /* Each item has a bit (index in user items array), discussed later 
> > */
> >  uitem = user_header->items[epi->bit];
> >
> >  if (!atomic_fetch_or(uitem->ready_events, pollflags)) {
> >      i = atomic_add(&ep->user_header->tail, 1);
>
> So this is where you increment tail
>
> >      item_idx = &user_index[i & index_mask];
> >
> >      /* Signal with a bit, user spins on index expecting value > 0 */
> >      *item_idx = idx + 1;
>
> IUC, this is where you write the idx into shared memory, which is
> _after_ tail has already been incremented.
>
> >  }
> >
> > Important thing here is that ring can't infinitely grow and corrupt 
> > other
> > elements, because kernel always checks that item was marked as ready, 
> > so
> > userspace has to clear ready_events field.
> >
> > On userside events the following code should be used in order to 
> > consume
> > events:
> >
> >  tail = READ_ONCE(header->tail);
> >  for (i = 0; header->head != tail; header->head++) {
> >      item_idx_ptr = &index[idx & indeces_mask];
> >
> >      /*
> >       * Spin here till we see valid index
> >       */
> >      while (!(idx = __atomic_load_n(item_idx_ptr, __ATOMIC_ACQUIRE)))
> >          ;
>
> Which you then try and fix up by busy waiting for @idx to become !0 ?!
>
> Why not write the idx first, then increment the ->tail, such that when
> we see ->tail, we already know idx must be correct?
>
> >      item = &header->items[idx - 1];
> >
> >      /*
> >       * Mark index as invalid, that is for userspace only, kernel does 
> > not care
> >       * and will refill this pointer only when observes that event is 
> > cleared,
> >       * which happens below.
> >       */
> >      *item_idx_ptr = 0;
>
> That avoids this store too.
>
> >      /*
> >       * Fetch data first, if event is cleared by the kernel we drop 
> > the data
> >       * returning false.
> >       */
> >      event->data = item->event.data;
> >      event->events = __atomic_exchange_n(&item->ready_events, 0,
> >                          __ATOMIC_RELEASE);
> >
> >  }
>
> Aside from that, you have to READ/WRITE_ONCE() on ->head, to avoid
> load/store tearing.

Yes, clear. Thanks.

> That would give something like:
>
> kernel:
>
> 	slot = atomic_fetch_inc(&ep->slot);
> 	item_idx = &user_index[slot & idx_mask];
> 	WRITE_ONCE(*item_idx, idx);
> 	smp_store_release(&ep->user_header->tail, slot);

This can't be called from many cpus,  tail can be overwritten with "old"
value.  That what I try to solve.

--
Roman