From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH bpf-next 3/3] bpf, libbpf: use proper barriers in perf
 ring buffer walk
Date: Wed, 17 Oct 2018 17:51:35 +0200
Message-ID: <20181017155135.GN3121@hirez.programming.kicks-ass.net>
References: <20181017144156.16639-1-daniel@iogearbox.net>
 <20181017144156.16639-4-daniel@iogearbox.net>
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        john.fastabend@gmail.com, netdev@vger.kernel.org
To: Daniel Borkmann <daniel@iogearbox.net>
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On Wed, Oct 17, 2018 at 04:41:56PM +0200, Daniel Borkmann wrote:
> +static __u64 bpf_perf_read_head(struct perf_event_mmap_page *header)
> +{
> +	__u64 data_head = READ_ONCE(header->data_head);
> +
> +	smp_rmb();
> +	return data_head;
> +}
> +
> +static void bpf_perf_write_tail(struct perf_event_mmap_page *header,
> +				__u64 data_tail)
> +{
> +	smp_mb();
> +	header->data_tail = data_tail;
> +}

Same coments, either smp_load_acquire()/smp_store_release() or at the
very least a WRITE_ONCE() there.