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Date:	Tue, 10 Nov 2009 11:27:42 +0100
From:	Peter Zijlstra <>
To:	Frederic Weisbecker <>
Cc:	Ingo Molnar <>, LKML <>,
	Masami Hiramatsu <>,
	Arnaldo Carvalho de Melo <>,
	Mike Galbraith <>,
	Paul Mackerras <>,
	Steven Rostedt <>,
	Li Zefan <>, Jason Baron <>
Subject: Re: [PATCH v2] tracing: Protect the buffer from recursion in perf

On Fri, 2009-11-06 at 04:13 +0100, Frederic Weisbecker wrote:
> While tracing using events with perf, if one enables the
> lockdep:lock_acquire event, it will infect every other perf trace
> events.
> Basically, you can enable whatever set of trace events through perf
> but if this event is part of the set, the only result we can get is a
> long list of lock_acquire events of rcu read lock, and only that.
> This is because of a recursion inside perf.
> 1) When a trace event is triggered, it will fill a per cpu buffer and
>    submit it to perf.
> 2) Perf will commit this event but will also protect some data using
>    rcu_read_lock
> 3) A recursion appears: rcu_read_lock triggers a lock_acquire event that
>    will fill the per cpu event and then submit the buffer to perf.
> 4) Perf detects a recursion and ignore it
> 5) Perf continue its work on the previous event, but its buffer has been
>    overwritten by the lock_acquire event, it has then been turned into
>    a lock_acquire event of rcu read lock
> Such scenario also happens with lock_release with rcu_read_unlock().
> We could turn the rcu_read_lock() into __rcu_read_lock() to drop the
> lock debugging from perf fast path, but that would make us lose
> the rcu debugging and that doesn't prevent from other possible kind of
> recursion from perf in the future.
> This patch adds a recursion protection based on a counter on the perf
> trace per cpu buffers to solve the problem.

There already is recursion protection in
kernel/perf_event.c:perf_swevent_recursion_context() and thereabouts.
Could you not fix this by widening its scope?
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