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From:   David Howells <dhowells@redhat.com>
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To:     Hillf Danton <hdanton@sina.com>
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Subject: Re: [PATCH 04/11] General notification queue with user mmap()'able ring buffer [ver #7]
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Date:   Tue, 03 Sep 2019 17:06:37 +0100
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Hillf Danton <hdanton@sina.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