linux-kernel - Re: [RFC PATCH 04/14] pipe: Add O_NOTIFICATION

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Date:   Fri, 08 Nov 2019 06:42:50 +0000
From:   David Howells <dhowells@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     dhowells@...hat.com,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>, raven@...maw.net,
        Christian Brauner <christian@...uner.io>,
        keyrings@...r.kernel.org, USB list <linux-usb@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 04/14] pipe: Add O_NOTIFICATION_PIPE [ver #2]

Andy Lutomirski <luto@...nel.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