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SubjectRe: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless
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Frederic Weisbecker <fweisbec@gmail.com> writes:
O

> _ The only race could happen between the current thread and an interrupt. If an
> interrupt is raised, it will increase the index of the return stack storage and
> then execute until the end of the tracing to finally free the index it used.
> We don't need to disable irqs.
>
> This is theorical. In practice, I've tested it with a two-core SMP and had no
> problem at all. Perhaps -tip testing could confirm it.

The problem I think is that you assume the ++ is atomic against
interrupts, which is not guaranteed by the C compiler. e.g.
it would be perfectly legal for the compiler to generate code like

local register i
i = index;
write to index'ed array using i
<--------- interrupt here would overwrite data
...
index = i + 1;
You would need to convert the index access to a "local_add_return()" and
possibly also add memory barriers.

-Andi

--
ak@linux.intel.com


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