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

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Date:	Thu, 16 Jul 2015 16:41:45 -0500
From:	Tom Zanussi <tom.zanussi@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
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, 2015-07-16 at 19:49 +0200, Peter Zijlstra wrote:
> 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.

Not assuming any magic properties - I just need an atomic 64-bit counter
for the sums and that's the API for setting/reading those.  When reading
a live trace the exact sum you get is kind of arbitrary..

Tom

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