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 07/13] epoll: call ep_add_event_to_uring() from ep_poll_callback()
Date: Fri, 31 May 2019 17:05:16 +0200 [thread overview]
Message-ID: <8dc64c770b693aeb2040cca7ec697a7a@suse.de> (raw)
In-Reply-To: <20190531130516.GA2606@hirez.programming.kicks-ass.net>
On 2019-05-31 15:05, Peter Zijlstra wrote:
> 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.
But it seems if we afraid of load tearing that should be fixed
separately,
independently of this patchset, because epi->event.events is updated
in ep_modify() and races with ep_poll_callback(), which reads the value
in couple of places.
Probably nothing terrible will happen, because eventually event comes
or just ignored.
> 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.
Locks, nice.
--
Roman
next prev parent reply other threads:[~2019-05-31 15:05 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
2019-05-31 15:05 ` Roman Penyaev [this message]
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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