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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Aleksa Sarai' <cyphar@cyphar.com>,
        Josh Triplett <josh@joshtriplett.org>
CC:     "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "io-uring@vger.kernel.org" <io-uring@vger.kernel.org>,
        "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Arnd Bergmann <arnd@arndb.de>, Jens Axboe <axboe@kernel.dk>
Subject: RE: [PATCH v4 2/3] fs: openat2: Extend open_how to allow
 userspace-selected fds
Thread-Topic: [PATCH v4 2/3] fs: openat2: Extend open_how to allow
 userspace-selected fds
Thread-Index: AQHWFjeEcgUetSiAV0WbnffNc+KTs6iAvc2Q
Date:   Sun, 19 Apr 2020 21:18:05 +0000
Message-ID: <7f02bf52254443e380c33cae7c1fd5f0@AcuMS.aculab.com>
References: <cover.1586830316.git.josh@joshtriplett.org>
 <f969e7d45a8e83efc1ca13d675efd8775f13f376.1586830316.git.josh@joshtriplett.org>
 <20200419104404.j4e5gxdn2duvmu6s@yavin.dot.cyphar.com>
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From: Aleksa Sarai
> Sent: 19 April 2020 11:44
> 
> 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)?

I suspect (there are no comments in the io_uring code to say what it does)
that the io_uring code uses a thread of the user process to sequentially
execute IO requests that the main application has added to a queue.

So it might make sense to queue up open/read/close.
But that ought to be within the io_uring code.

	David

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