tracing/function-return-tracer: make the function return tracer lockless

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Impact: remove spinlocks and irq disabling in function return tracer.

I've tried to figure out all of the race condition that could happen
when the tracer pushes or pops a return address trace to/from the
current thread_info.

Theory:

_ One thread can only execute on one cpu at a time. So this code
  doesn't need to be SMP-safe. Just drop the spinlock.

_ 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.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

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This commit is contained in:

Frederic Weisbecker 2008-11-12 22:47:54 +01:00 • committed by Ingo Molnar

parent cb9382e5a9

commit 62d59d17a5

1 changed files with 5 additions and 38 deletions

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arch/x86/kernel/ftrace.c

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