linux-kernel - Re: [PATCH 3/5] bpf: Add bpf_user_ringbuf

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Message-ID: <Yv1XoPoICceMkjH4@maniforge.dhcp.thefacebook.com>
Date:   Wed, 17 Aug 2022 16:03:28 -0500
From:   David Vernet <void@...ifault.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, john.fastabend@...il.com, martin.lau@...ux.dev,
        song@...nel.org, yhs@...com, kpsingh@...nel.org, sdf@...gle.com,
        haoluo@...gle.com, jolsa@...nel.org, tj@...nel.org,
        joannelkoong@...il.com, linux-kernel@...r.kernel.org,
        Kernel-team@...com
Subject: Re: [PATCH 3/5] bpf: Add bpf_user_ringbuf_drain() helper

On Wed, Aug 17, 2022 at 03:24:40PM -0500, David Vernet wrote:

> [...]
> > right, I think __atomic_store_n() can be used in libbpf for this with
> > seq_cst ordering
> 
> __atomic_store_n(__ATOMIC_SEQ_CST) will do the correct thing on x86, but it
> is not guaranteed to provide a full acq/rel barrier according to the C
> standard. __atomic_store_n(__ATOMIC_SEQ_CST) means "store-release, and also
> participates in the sequentially-consistent global ordering".
> 
> I believe we actually need an __atomic_store_n(__ATOMIC_ACQ_REL) here. I

Sorry, I meant __atomic_exchange_n() rather than __atomic_store_n().