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Date:   Thu, 16 May 2019 11:37:47 -0700
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Subject: Re: [PATCH] perf/ring_buffer: Fix exposing a temporarily decreased data_head.
From:   Yabin Cui <yabinc@google.com>
To:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>
Cc:     linux-kernel@vger.kernel.org, Yabin Cui <yabinc@google.com>
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>Indeed, good catch! Have you observed this, or is this patch due to code
inspection?

I observed this. I was using perf_event_open instead of linux tools perf
to monitor a process continuously forking new processes, generating a lot
of mmap records. And a sample record happens while generating a mmap record.

For missing barriers, I thought local_xxx provided implied barriers. But
I found atomic_ops.txt said not. Thanks for reminding! I will add them in
path v2.

I also think it's a good idea to use READ_ONCE/WRITE_ONCE for rb->nest. Will
use it in patch v2.