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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 15:05:16 +0200	[thread overview]
Message-ID: <20190531130516.GA2606@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <98971429dc36e8a2e3417af1744de2b2@suse.de>

On Fri, May 31, 2019 at 01:22:54PM +0200, Roman Penyaev wrote:
> On 2019-05-31 11:56, Peter Zijlstra wrote:
> > On Thu, May 16, 2019 at 10:58:04AM +0200, Roman Penyaev wrote:

> > > +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.
> 
> Yes, we race with the current function and with ep_modify().  Then,
> ep_modify()
> should do something as the following:
> 
> -	epi->event.events = event->events
> +	xchg(&epi->event.events, event->events);
> 
> Is that ok?

That should be correct, but at that point I think we should also always
read the thing with READ_ONCE() to avoid load-tearing. And I suspect it
then becomes sensible to change the type to atomic_t.

atomic_set() vs atomic_cmpxchg() only carries the extra overhead on
those 'dodgy' platforms.

> Just curious: what are these archs?

Oh, lovely stuff like parisc, sparc32 and arc-eznps. See
arch/parisc/lib/bitops.c:__cmpxchg_*() for example :/ Those systems only
have a single truly atomic op (something from the xchg / test-and-set
family) and the rest is fudged on top of that.


  reply	other threads:[~2019-05-31 13:05 UTC|newest]

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
2019-05-31 11:22     ` Roman Penyaev
2019-05-31 13:05       ` Peter Zijlstra [this message]
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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