From: Aleksa Sarai <cyphar@cyphar.com>
To: Jens Axboe <axboe@kernel.dk>
Cc: Josh Triplett <josh@joshtriplett.org>,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
io-uring@vger.kernel.org, linux-arch@vger.kernel.org,
Alexander Viro <viro@zeniv.linux.org.uk>,
Arnd Bergmann <arnd@arndb.de>
Subject: Re: [PATCH v4 2/3] fs: openat2: Extend open_how to allow userspace-selected fds
Date: Mon, 20 Apr 2020 12:06:42 +1000 [thread overview]
Message-ID: <20200420020642.xzkqikia6kmslkjh@yavin.dot.cyphar.com> (raw)
In-Reply-To: <b7dae79b-4c5f-65f6-0960-617070357201@kernel.dk>
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On 2020-04-19, Jens Axboe <axboe@kernel.dk> wrote:
> On 4/19/20 4:44 AM, Aleksa Sarai wrote:
> > On 2020-04-13, Josh Triplett <josh@joshtriplett.org> wrote:
> >> Inspired by the X protocol's handling of XIDs, allow userspace to select
> >> the file descriptor opened by openat2, so that it can use the resulting
> >> file descriptor in subsequent system calls without waiting for the
> >> response to openat2.
> >>
> >> In io_uring, this allows sequences like openat2/read/close without
> >> waiting for the openat2 to complete. Multiple such sequences can
> >> overlap, as long as each uses a distinct file descriptor.
> >
> > I'm not sure I understand this explanation -- how can you trigger a
> > syscall with an fd that hasn't yet been registered (unless you're just
> > hoping the race goes in your favour)?
>
> io_uring can do chains of requests, where each link in the chain isn't
> started until the previous one has completed. Hence if you know what fd
> that openat2 will return, you can submit a chain ala:
>
> <open file X, give me fd Y><read from fd Y><close fd Y>
>
> as a single submission. This isn't possible to do currently, as the read
> will depend on the output of the open, and we have no good way of
> knowing what that fd will be.
Ah! I was aware of io_uring's chaining feature but thought it had access
to the return of the previous stage -- now this makes much more sense.
Thanks.
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
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next prev parent reply other threads:[~2020-04-20 2:07 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-14 2:14 [PATCH v4 0/3] Support userspace-selected fds Josh Triplett
2020-04-14 2:15 ` [PATCH v4 1/3] fs: Support setting a minimum fd for "lowest available fd" allocation Josh Triplett
2020-04-14 2:15 ` [PATCH v4 2/3] fs: openat2: Extend open_how to allow userspace-selected fds Josh Triplett
2020-04-19 10:44 ` Aleksa Sarai
2020-04-19 21:18 ` David Laight
2020-04-19 22:22 ` Jens Axboe
2020-04-20 2:06 ` Aleksa Sarai [this message]
2020-04-20 21:14 ` Josh Triplett
[not found] ` <f969e7d45a8e83efc1ca13d675efd8775f13f376.1586830316.git.josh-iaAMLnmF4UmaiuxdJuQwMA@public.gmane.org>
2020-04-27 13:52 ` [fs] ce436509a8: ltp.openat203.fail kernel test robot
2020-04-27 13:52 ` kernel test robot
2020-04-27 13:52 ` [LTP] " kernel test robot
2020-04-27 14:27 ` Cyril Hrubis
2020-04-27 14:27 ` Cyril Hrubis
2020-04-27 14:27 ` Cyril Hrubis
2020-04-28 0:51 ` Aleksa Sarai
2020-04-28 0:51 ` Aleksa Sarai
2020-04-28 0:51 ` Aleksa Sarai
2020-04-28 15:30 ` Cyril Hrubis
2020-04-28 15:30 ` Cyril Hrubis
2020-04-28 15:30 ` Cyril Hrubis
2020-04-28 15:30 ` Cyril Hrubis
2020-04-28 15:35 ` [LTP] " Aleksa Sarai
2020-04-28 15:35 ` Aleksa Sarai
2020-04-28 15:35 ` Aleksa Sarai
2020-04-14 2:16 ` [PATCH v4 3/3] fs: pipe2: Support O_SPECIFIC_FD Josh Triplett
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