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 18:53:49 +0200 [thread overview]
Message-ID: <20190531165349.GF2606@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <ef6cb59e319f185619e531c0a39bd32a@suse.de>
On Fri, May 31, 2019 at 04:28:36PM +0200, Roman Penyaev wrote:
> On 2019-05-31 14:53, Peter Zijlstra wrote:
> > On Fri, May 31, 2019 at 01:15:21PM +0200, Roman Penyaev wrote:
> > > On 2019-05-31 11:56, Peter Zijlstra wrote:
> > > > On Thu, May 16, 2019 at 10:58:03AM +0200, Roman Penyaev wrote:
> >
> > > > > + i = __atomic_fetch_add(&ep->user_header->tail, 1,
> > > > > + __ATOMIC_ACQUIRE);
> > > >
> > > > afaict __atomic_fetch_add() does not exist.
> > >
> > > That is gcc extension. I did not find any API just to increment
> > > the variable atomically without using/casting to atomic. What
> > > is a proper way to achieve that?
> >
> > That's C11 atomics, and those shall not be used in the kernel. For one
> > they're not available in the minimally required GCC version (4.6).
> >
> > The proper and only way is to use atomic_t, but also you cannot share
> > atomic_t with userspace.
>
> Yes, that what I tried to avoid choosing c11 extension.
>
> >
> > The normal way of doing something like this is to have a kernel private
> > atomic_t and copy the value out to userspace using smp_store_release().
>
> Since this path is lockless unfortunately that won't work. So seems
> the only way is to do one more cmpxchg (sigh) or give up and take a
> look (sad sigh).
Of course it works; most of the extant buffers already have a shadow
tail and do the update to userspace with a store-release.
next prev parent reply other threads:[~2019-05-31 16:53 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 [this message]
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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