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Date: Mon, 10 Jul 2017 12:00:08 -0400
From: "Emilio G. Cota" <cota@braap.org>
Message-ID: <20170710160008.GB1216@flamenco>
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Subject: Re: [Qemu-devel] [PATCH 17/22] tcg: distribute profiling counters
 across TCGContext's
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To: Richard Henderson <rth@twiddle.net>
Cc: qemu-devel@nongnu.org

On Sun, Jul 09, 2017 at 11:44:10 -1000, Richard Henderson wrote:
> On 07/09/2017 11:14 AM, Emilio G. Cota wrote:
> >On Sun, Jul 09, 2017 at 10:45:55 -1000, Richard Henderson wrote:
> >>On 07/08/2017 09:50 PM, Emilio G. Cota wrote:
> >>>+    /* includes aborted translations because of exceptions */
> >>>+    atomic_set(&prof->tb_count1, prof->tb_count1 + 1);
> >>
> >>Again, atomic_set without atomic_read is pointless.
> >>Either you're trying to give the compiler extra information, or you aren't.
> >
> >See my comment to patch 3.
> 
> I still disagree.  It's Just Plain Confusing.

To me,
  atomic_set(&foo, foo + 1)
and
  atomic_set(&foo, atomic_read(&foo) + 1)
read differently.

The former tells me that no other thread can update the value.

The latter makes me presume that other threads *can* update the value,
which makes me wonder whether this is a bug and we should use atomic_inc(),
or perhaps we are OK with missing counts.

		Emilio