linux-kernel - Re: [PATCH 04/11] General notification queue with user mmap()'able ring buffer [ver #7]

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Message-ID: <7403.1567526797@warthog.procyon.org.uk>
Date:   Tue, 03 Sep 2019 17:06:37 +0100
From:   David Howells <dhowells@...hat.com>
To:     Hillf Danton <hdanton@...a.com>
Cc:     dhowells@...hat.com, viro@...iv.linux.org.uk,
        Casey Schaufler <casey@...aufler-ca.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        nicolas.dichtel@...nd.com, raven@...maw.net,
        Christian Brauner <christian@...uner.io>,
        keyrings@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/11] General notification queue with user mmap()'able ring buffer [ver #7]

Hillf Danton <hdanton@...a.com> wrote:

> > +	smp_store_release(&buf->meta.head, head);
> 
> Add a line of comment for the paring smp_load_acquire().
> I did not find it in 04/11.

You won't find smp_load_acquire() - it's not in the kernel, though if you look
in the sample, you'll find the corresponding barrier in userspace.  Note that
there's a further implicit barrier you don't see.

I've added the comments:

	/* Barrier against userspace, ordering data read before tail read */
	ring_tail = READ_ONCE(buf->meta.tail);

and:

	/* Barrier against userspace, ordering head update after data write. */
	smp_store_release(&buf->meta.head, head);

David