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From: Linus Torvalds <torvalds@linux-foundation.org>
To: David Howells <dhowells@redhat.com>
Cc: Ray Strode <rstrode@redhat.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Steven Whitehouse <swhiteho@redhat.com>,
	Nicolas Dichtel <nicolas.dichtel@6wind.com>,
	raven@themaw.net, keyrings@vger.kernel.org,
	linux-usb@vger.kernel.org,
	linux-block <linux-block@vger.kernel.org>,
	Christian Brauner <christian@brauner.io>,
	LSM List <linux-security-module@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	"Ray, Debarshi" <debarshi.ray@gmail.com>,
	Robbie Harwood <rharwood@redhat.com>
Subject: Re: Why add the general notification queue and its sources
Date: Fri, 6 Sep 2019 08:53:09 -0700
Message-ID: <CAHk-=wioHmz69394xKRqFkhK8si86P_704KgcwjKxawLAYAiug@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wiR1fpahgKuxSOQY6OfgjWD+MKz8UF6qUQ6V_y2TC_V6w@mail.gmail.com>

On Fri, Sep 6, 2019 at 8:35 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> This is why I like pipes. You can use them today. They are simple, and
> extensible, and you don't need to come up with a new subsystem and
> some untested ad-hoc thing that nobody has actually used.

The only _real_ complexity is to make sure that events are reliably parseable.

That's where you really want to use the Linux-only "packet pipe"
thing, becasue otherwise you have to have size markers or other things
to delineate events. But if you do that, then it really becomes
trivial.

And I checked, we made it available to user space, even if the
original reason for that code was kernel-only autofs use: you just
need to make the pipe be O_DIRECT.

This overly stupid program shows off the feature:

        #define _GNU_SOURCE
        #include <fcntl.h>
        #include <unistd.h>

        int main(int argc, char **argv)
        {
                int fd[2];
                char buf[10];

                pipe2(fd, O_DIRECT | O_NONBLOCK);
                write(fd[1], "hello", 5);
                write(fd[1], "hi", 2);
                read(fd[0], buf, sizeof(buf));
                read(fd[0], buf, sizeof(buf));
                return 0;
        }

and it you strace it (because I was too lazy to add error handling or
printing of results), you'll see

    write(4, "hello", 5)                    = 5
    write(4, "hi", 2)                       = 2
    read(3, "hello", 10)                    = 5
    read(3, "hi", 10)                       = 2

note how you got packets of data on the reader side, instead of
getting the traditional "just buffer it as a stream".

So now you can even have multiple readers of the same event pipe, and
packetization is obvious and trivial. Of course, I'm not sure why
you'd want to have multiple readers, and you'd lose _ordering_, but if
all events are independent, this _might_ be a useful thing in a
threaded environment. Maybe.

(Side note: a zero-sized write will not cause a zero-sized packet. It
will just be dropped).

               Linus

  reply index

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-04 22:15 [PATCH 00/11] Keyrings, Block and USB notifications [ver #8] David Howells
2019-09-04 22:15 ` [PATCH 01/11] uapi: General notification ring definitions " David Howells
2019-09-04 22:16 ` [PATCH 02/11] security: Add hooks to rule on setting a watch " David Howells
2019-09-04 22:16 ` [PATCH 03/11] security: Add a hook for the point of notification insertion " David Howells
2019-09-04 22:16 ` [PATCH 04/11] General notification queue with user mmap()'able ring buffer " David Howells
2019-09-04 22:16 ` [PATCH 05/11] keys: Add a notification facility " David Howells
2019-09-04 22:16 ` [PATCH 06/11] Add a general, global device notification watch list " David Howells
2019-09-04 22:16 ` [PATCH 07/11] block: Add block layer notifications " David Howells
2019-09-04 22:16 ` [PATCH 08/11] usb: Add USB subsystem " David Howells
2019-09-04 22:17 ` [PATCH 09/11] Add sample notification program " David Howells
2019-09-04 22:17 ` [PATCH 10/11] selinux: Implement the watch_key security hook " David Howells
2019-09-04 22:17 ` [PATCH 11/11] smack: Implement the watch_key and post_notification hooks " David Howells
2019-09-04 22:28 ` [PATCH 00/11] Keyrings, Block and USB notifications " Linus Torvalds
2019-09-05 17:01 ` Why add the general notification queue and its sources David Howells
2019-09-05 17:19   ` Linus Torvalds
2019-09-05 18:32     ` Ray Strode
2019-09-05 20:39       ` Linus Torvalds
2019-09-06 19:32         ` Ray Strode
2019-09-06 19:41           ` Ray Strode
2019-09-06 19:53           ` Robbie Harwood
2019-09-05 21:32       ` David Howells
2019-09-05 22:08         ` Linus Torvalds
2019-09-05 23:18         ` David Howells
2019-09-06  0:07           ` Linus Torvalds
2019-09-06 10:09           ` David Howells
2019-09-06 15:35             ` Linus Torvalds
2019-09-06 15:53               ` Linus Torvalds [this message]
2019-09-06 16:12                 ` Steven Whitehouse
2019-09-06 17:07                   ` Linus Torvalds
2019-09-06 17:14                     ` Linus Torvalds
2019-09-06 21:19                       ` David Howells
2019-09-06 17:14                   ` Andy Lutomirski
2019-09-05 18:37     ` Steven Whitehouse
2019-09-05 18:51       ` Ray Strode
2019-09-05 20:09         ` David Lehman
2019-09-05 18:33   ` Greg Kroah-Hartman

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