linux-kernel - Re: [PATCH v9 07/22] tracing: Add lock-free tracing

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Date:	Thu, 16 Jul 2015 19:49:46 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tom Zanussi <tom.zanussi@...ux.intel.com>
Cc:	rostedt@...dmis.org, daniel.wagner@...-carit.de,
	masami.hiramatsu.pt@...achi.com, namhyung@...nel.org,
	josh@...htriplett.org, andi@...stfloor.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 07/22] tracing: Add lock-free tracing_map

On Thu, Jul 16, 2015 at 12:22:40PM -0500, Tom Zanussi wrote:
> +	for (i = 0; i < elt->map->n_fields; i++) {
> +		atomic64_set(&dup_elt->fields[i].sum,
> +			     atomic64_read(&elt->fields[i].sum));
> +		dup_elt->fields[i].cmp_fn = elt->fields[i].cmp_fn;
> +	}
> +
> +	return dup_elt;
> +}

So there is a lot of atomic64_{set,read}() in this patch set, what kind
of magic properties do you assume they have?

Note that atomic*_{set,read}() are weaker than {WRITE,READ}_ONCE(), so
if you're assuming they do that, you're mistaken -- although it is on a
TODO list someplace to go fix that.
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