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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 07/13] epoll: call ep_add_event_to_uring() from ep_poll_callback()
Date: Fri, 31 May 2019 11:56:16 +0200
Message-ID: <20190531095616.GD17637@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20190516085810.31077-8-rpenyaev@suse.de>

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@suse.de>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: linux-fsdevel@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> 
> 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.


  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
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 [this message]
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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