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From: Andy Lutomirski <luto@kernel.org>
To: David Howells <dhowells@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Casey Schaufler <casey@schaufler-ca.com>,
	Stephen Smalley <sds@tycho.nsa.gov>,
	Nicolas Dichtel <nicolas.dichtel@6wind.com>,
	raven@themaw.net, Christian Brauner <christian@brauner.io>,
	keyrings@vger.kernel.org, USB list <linux-usb@vger.kernel.org>,
	linux-block <linux-block@vger.kernel.org>,
	LSM List <linux-security-module@vger.kernel.org>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH 04/14] pipe: Add O_NOTIFICATION_PIPE [ver #2]
Date: Thu, 7 Nov 2019 21:06:53 -0800
Message-ID: <CALCETrWeN9CGJHz0dzG1uH5Qjbr+xG3OKZuEd33eBY_rAzVkqQ@mail.gmail.com> (raw)
In-Reply-To: <6964.1573152517@warthog.procyon.org.uk>

On Thu, Nov 7, 2019 at 10:48 AM David Howells <dhowells@redhat.com> wrote:
>
> Andy Lutomirski <luto@kernel.org> wrote:
>
> > > Add an O_NOTIFICATION_PIPE flag that can be passed to pipe2() to indicate
> > > that the pipe being created is going to be used for notifications.  This
> > > suppresses the use of splice(), vmsplice(), tee() and sendfile() on the
> > > pipe as calling iov_iter_revert() on a pipe when a kernel notification
> > > message has been inserted into the middle of a multi-buffer splice will be
> > > messy.
> >
> > How messy?
>
> Well, iov_iter_revert() on a pipe iterator simply walks backwards along the
> ring discarding the last N contiguous slots (where N is normally the number of
> slots that were filled by whatever operation is being reverted).
>
> However, unless the code that transfers stuff into the pipe takes the spinlock
> spinlock and disables softirqs for the duration of its ring filling, what were
> N contiguous slots may now have kernel notifications interspersed - even if it
> has been holding the pipe mutex.
>
> So, now what do you do?  You have to free up just the buffers relevant to the
> iterator and then you can either compact down the ring to free up the space or
> you can leave null slots and let the read side clean them up, thereby
> reducing the capacity of the pipe temporarily.
>
> Either way, iov_iter_revert() gets more complex and has to hold the spinlock.

I feel like I'm missing something fundamental here.

I can open a normal pipe from userspace (with pipe() or pipe2()), and
I can have two threads.  One thread writes to the pipe with write().
The other thread writes with splice().  Everything works fine.  What's
special about notifications?

  reply index

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-07 13:35 [RFC PATCH 00/14] pipe: Keyrings, Block and USB notifications " David Howells
2019-11-07 13:35 ` [RFC PATCH 01/14] uapi: General notification queue definitions " David Howells
2019-11-07 13:35 ` [RFC PATCH 02/14] security: Add hooks to rule on setting a watch " David Howells
2019-11-07 13:35 ` [RFC PATCH 03/14] security: Add a hook for the point of notification insertion " David Howells
2019-11-07 13:35 ` [RFC PATCH 04/14] pipe: Add O_NOTIFICATION_PIPE " David Howells
2019-11-07 18:16   ` Andy Lutomirski
2019-11-07 18:48   ` David Howells
2019-11-08  5:06     ` Andy Lutomirski [this message]
2019-11-08  6:42     ` David Howells
2019-11-07 13:36 ` [RFC PATCH 05/14] pipe: Add general notification queue support " David Howells
2019-11-07 13:36 ` [RFC PATCH 06/14] keys: Add a notification facility " David Howells
2019-11-07 13:36 ` [RFC PATCH 07/14] Add sample notification program " David Howells
2019-11-07 13:36 ` [RFC PATCH 08/14] pipe: Allow buffers to be marked read-whole-or-error for notifications " David Howells
2019-11-07 18:15   ` Andy Lutomirski
2019-11-07 18:23   ` David Howells
2019-11-07 13:36 ` [RFC PATCH 09/14] pipe: Add notification lossage handling " David Howells
2019-11-07 13:36 ` [RFC PATCH 10/14] Add a general, global device notification watch list " David Howells
2019-11-07 13:37 ` [RFC PATCH 11/14] block: Add block layer notifications " David Howells
2019-11-07 13:37 ` [RFC PATCH 12/14] usb: Add USB subsystem " David Howells
2019-11-07 13:37 ` [RFC PATCH 13/14] selinux: Implement the watch_key security hook " David Howells
2019-11-07 13:37 ` [RFC PATCH 14/14] smack: Implement the watch_key and post_notification hooks " David Howells
2019-11-07 17:16 ` [RFC PATCH 05/14] pipe: Add general notification queue support " David Howells

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