Re: [PATCH v3 06/13] epoll: introduce helpers for adding/removing events to uring

Roman Penyaev
2019-05-31T14:21:34Z

On 2019-05-31 14:56, Peter Zijlstra wrote:
> On Fri, May 31, 2019 at 01:15:21PM +0200, Roman Penyaev wrote:
>> On 2019-05-31 11:56, Peter Zijlstra wrote:
>> > On Thu, May 16, 2019 at 10:58:03AM +0200, Roman Penyaev wrote:
>> > > +static inline bool ep_add_event_to_uring(struct epitem *epi,
>> > > __poll_t pollflags)
>> > > +{
>> > > + struct eventpoll *ep = epi->ep;
>> > > + struct epoll_uitem *uitem;
>> > > + bool added = false;
>> > > +
>> > > + if (WARN_ON(!pollflags))
>> > > + return false;
>> > > +
>> > > + uitem = &ep->user_header->items[epi->bit];
>> > > + /*
>> > > + * Can be represented as:
>> > > + *
>> > > + * was_ready = uitem->ready_events;
>> > > + * uitem->ready_events &= ~EPOLLREMOVED;
>> > > + * uitem->ready_events |= pollflags;
>> > > + * if (!was_ready) {
>> > > + * // create index entry
>> > > + * }
>> > > + *
>> > > + * See the big comment inside ep_remove_user_item(), why it is
>> > > + * important to mask EPOLLREMOVED.
>> > > + */
>> > > + if (!atomic_or_with_mask(&uitem->ready_events,
>> > > + pollflags, EPOLLREMOVED)) {
>> > > + unsigned int i, *item_idx, index_mask;
>> > > +
>> > > + /*
>> > > + * Item was not ready before, thus we have to insert
>> > > + * new index to the ring.
>> > > + */
>> > > +
>> > > + index_mask = ep_max_index_nr(ep) - 1;
>> > > + i = __atomic_fetch_add(&ep->user_header->tail, 1,
>> > > + __ATOMIC_ACQUIRE);
>> > > + item_idx = &ep->user_index[i & index_mask];
>> > > +
>> > > + /* Signal with a bit, which is > 0 */
>> > > + *item_idx = epi->bit + 1;
>> >
>> > Did you just increment the user visible tail pointer before you filled
>> > the data? That is, can the concurrent userspace observe the increment
>> > before you put credible data in its place?
>>
>> No, the "data" is the "ready_events" mask, which was updated before,
>> using cmpxchg, atomic_or_with_mask() call. All I need is to put an
>> index of just updated item to the uring.
>>
>> Userspace, in its turn, gets the index from the ring and then checks
>> the mask.
>
> But where do you write the index into the shared memory? That index
> should be written before you publish the new tail.

The ep_add_event_to_uring() is lockless, thus I can't increase tail
after,
I need to reserve the index slot, where to write to. I can use shadow
tail,
which is not seen by userspace, but I have to guarantee that tail is
updated
with shadow tail *after* all callers of ep_add_event_to_uring() are
left.
That is possible, please see the code below, but it adds more
complexity:

(code was tested on user side, thus has c11 atomics)

static inline void add_event__kernel(struct ring *ring, unsigned bit)
{
unsigned i, cntr, commit_cntr, *item_idx, tail, old;

i = __atomic_fetch_add(&ring->cntr, 1, __ATOMIC_ACQUIRE);
item_idx = &ring->user_itemsindex[i % ring->nr];

/* Update data */
*item_idx = bit;

commit_cntr = __atomic_add_fetch(&ring->commit_cntr, 1,
__ATOMIC_RELEASE);

tail = ring->user_header->tail;
rmb();
do {
cntr = ring->cntr;
if (cntr != commit_cntr)
/* Someone else will advance tail */
break;

old = tail;

} while ((tail =
__sync_val_compare_and_swap(&ring->user_header->tail, old, cntr)) !=
old);
}

Another way (current solution) is to spin on userspace side in order to
get
index > 0 (valid index is always > 0), i.e.:

item_idx_ptr = &index[idx & indeces_mask];

/*
* Spin here till we see valid index
*/
while (!(idx = __atomic_load_n(item_idx_ptr, __ATOMIC_ACQUIRE)))
;

So of course tail can be updated after, like you mentioned, but then I
have
to introduce locks. I want to keep it lockless on hot event path.

--
Roman