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From: Aleksa Sarai <[email protected]>
To: Jens Axboe <[email protected]>
Cc: Josh Triplett <[email protected]>,
	[email protected], [email protected],
	[email protected], [email protected],
	Alexander Viro <[email protected]>,
	Arnd Bergmann <[email protected]>
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: <[email protected]> (raw)
In-Reply-To: <[email protected]>

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On 2020-04-19, Jens Axboe <[email protected]> wrote:
> On 4/19/20 4:44 AM, Aleksa Sarai wrote:
> > On 2020-04-13, Josh Triplett <[email protected]> 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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  reply	other threads:[~2020-04-20  2:07 UTC|newest]

Thread overview: 13+ 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]   ` <20200427135210.GB5770@shao2-debian>
2020-04-27 14:27     ` [LTP] [fs] ce436509a8: ltp.openat203.fail Cyril Hrubis
2020-04-28  0:51       ` Aleksa Sarai
2020-04-28 15:30         ` Cyril Hrubis
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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