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From: Peter Zijlstra <peterz@infradead.org>
To: Roman Penyaev <rpenyaev@suse.de>
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 06/13] epoll: introduce helpers for adding/removing events to uring
Date: Fri, 31 May 2019 11:56:07 +0200
Message-ID: <20190531095607.GC17637@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20190516085810.31077-7-rpenyaev@suse.de>

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);

afaict __atomic_fetch_add() does not exist.

> +		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?

> +
> +		/*
> +		 * Want index update be flushed from CPU write buffer and
> +		 * immediately visible on userspace side to avoid long busy
> +		 * loops.
> +		 */
> +		smp_wmb();

That's still complete nonsense.

> +
> +		added = true;
> +	}
> +
> +	return added;
> +}

  parent reply index

Thread overview: 50+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-16  8:57 [PATCH v3 00/13] epoll: support pollable epoll from userspace 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 [this message]
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

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