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From: David Howells <dhowells@redhat.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: dhowells@redhat.com,
	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: Fri, 08 Nov 2019 06:42:50 +0000
Message-ID: <7772.1573195370@warthog.procyon.org.uk> (raw)
In-Reply-To: <CALCETrWeN9CGJHz0dzG1uH5Qjbr+xG3OKZuEd33eBY_rAzVkqQ@mail.gmail.com>

Andy Lutomirski <luto@kernel.org> wrote:

> 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.

Yes.  Every operation you do on a pipe from userspace is serialised with the
pipe mutex - and both ends share the same pipe.

> What's special about notifications?

The post_notification() cannot take the pipe mutex.  It has to be callable
from softirq context.  Linus's idea is that when you're actually altering the
ring pointers you should hold the wake-queue spinlock, and post_notification()
holds the wake queue spinlock for the duration of the operation.

This means that post_notification() can be writing to the pipe whilst a
userspace-invoked operation is holding the pipe mutex and is also doing
something to the ring.

David


  parent 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
2019-11-08  6:42     ` David Howells [this message]
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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