netdev - Re: [PATCH bpf-next] selftests/bpf: fix sample

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Message-ID: <CAPhsuW5OH9paDZpG-KfYK3EkwpaQWPOcD6c0kyLQ7+ePs9Xd8g@mail.gmail.com>
Date:   Tue, 2 Jun 2020 09:18:16 -0700
From:   Song Liu <song@...nel.org>
To:     Andrii Nakryiko <andriin@...com>
Cc:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next] selftests/bpf: fix sample_cnt shared between two threads

On Mon, Jun 1, 2020 at 10:04 PM Andrii Nakryiko <andriin@...com> wrote:
>
> Make sample_cnt volatile to fix possible selftests failure due to compiler
> optimization preventing latest sample_cnt value to be visible to main thread.
> sample_cnt is incremented in background thread, which is then joined into main
> thread. So in terms of visibility sample_cnt update is ok. But because it's
> not volatile, compiler might make optimizations that would prevent main thread
> to see latest updated value. Fix this by marking global variable volatile.
>
> Fixes: cb1c9ddd5525 ("selftests/bpf: Add BPF ringbuf selftests")
> Signed-off-by: Andrii Nakryiko <andriin@...com>

Acked-by: Song Liu <songliubraving@...com>