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From: Roman Penyaev <rpenyaev@suse.de>
To: Peter Zijlstra <peterz@infradead.org>
Cc: azat@libevent.org, akpm@linux-foundation.org,
	viro@zeniv.linux.org.uk, torvalds@linux-foundation.org,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 00/13] epoll: support pollable epoll from userspace
Date: Fri, 31 May 2019 20:50:44 +0200
Message-ID: <327c990a4418b3d9c5c94787a37350bb@suse.de> (raw)
In-Reply-To: <20190531163312.GW2650@hirez.programming.kicks-ass.net>

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


      reply index

Thread overview: 50+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-16  8:57 Roman Penyaev
2019-05-16  8:57 ` [PATCH v3 01/13] epoll: move private helpers from a header to the source Roman Penyaev
2019-05-16  8:57 ` [PATCH v3 02/13] epoll: introduce user structures for polling from userspace Roman Penyaev
2019-05-16  8:58 ` [PATCH v3 03/13] epoll: allocate user header and user events ring " Roman Penyaev
2019-05-16  8:58 ` [PATCH v3 04/13] epoll: some sanity flags checks for epoll syscalls " Roman Penyaev
2019-05-16  8:58 ` [PATCH v3 05/13] epoll: offload polling to a work in case of epfd polled " Roman Penyaev
2019-05-21  7:51   ` Eric Wong
2019-05-22 12:54     ` Roman Penyaev
2019-05-16  8:58 ` [PATCH v3 06/13] epoll: introduce helpers for adding/removing events to uring Roman Penyaev
2019-05-31  9:55   ` Peter Zijlstra
2019-05-31 11:24     ` Roman Penyaev
2019-05-31 13:11       ` Peter Zijlstra
2019-05-31  9:56   ` Peter Zijlstra
2019-05-31 11:15     ` Roman Penyaev
2019-05-31 12:53       ` Peter Zijlstra
2019-05-31 14:28         ` Roman Penyaev
2019-05-31 16:53           ` Peter Zijlstra
2019-05-31 12:56       ` Peter Zijlstra
2019-05-31 14:21         ` Roman Penyaev
2019-05-31 16:51           ` Peter Zijlstra
2019-05-31 18:58             ` Roman Penyaev
2019-06-03  9:09               ` Peter Zijlstra
2019-06-03 10:02                 ` Roman Penyaev
2019-05-16  8:58 ` [PATCH v3 07/13] epoll: call ep_add_event_to_uring() from ep_poll_callback() Roman Penyaev
2019-05-31  9:56   ` Peter Zijlstra
2019-05-31 11:22     ` Roman Penyaev
2019-05-31 13:05       ` Peter Zijlstra
2019-05-31 15:05         ` Roman Penyaev
2019-05-16  8:58 ` [PATCH v3 08/13] epoll: support polling from userspace for ep_insert() Roman Penyaev
2019-05-16  8:58 ` [PATCH v3 09/13] epoll: support polling from userspace for ep_remove() Roman Penyaev
2019-05-16  8:58 ` [PATCH v3 10/13] epoll: support polling from userspace for ep_modify() Roman Penyaev
2019-05-16  8:58 ` [PATCH v3 11/13] epoll: support polling from userspace for ep_poll() Roman Penyaev
2019-05-16  8:58 ` [PATCH v3 12/13] epoll: support mapping for epfd when polled from userspace Roman Penyaev
2019-05-16  8:58 ` [PATCH v3 13/13] epoll: implement epoll_create2() syscall Roman Penyaev
2019-05-16 10:03   ` Arnd Bergmann
2019-05-16 10:20     ` Roman Penyaev
2019-05-16 10:57       ` Arnd Bergmann
2019-05-22  2:33       ` Andrew Morton
2019-05-22  9:11         ` Roman Penyaev
2019-05-22 11:14         ` Arnd Bergmann
2019-05-22 18:36           ` Andrew Morton
2019-05-31  9:55 ` [PATCH v3 00/13] epoll: support pollable epoll from userspace Peter Zijlstra
2019-05-31 14:48 ` Jens Axboe
2019-05-31 16:02   ` Roman Penyaev
2019-05-31 16:54     ` Jens Axboe
2019-05-31 19:45       ` Roman Penyaev
2019-05-31 21:09         ` Jens Axboe
2019-06-05  6:17           ` Roman Penyaev
2019-05-31 16:33 ` Peter Zijlstra
2019-05-31 18:50   ` Roman Penyaev [this message]

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