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Subject: Re: [PATCH 2/2] tracing/events/lockdep: move tracepoints within
 recursive protection
From: Peter Zijlstra <peterz@infradead.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@elte.hu>,
       Andrew Morton <akpm@linux-foundation.org>,
       Thomas Gleixner <tglx@linutronix.de>,
       Frederic Weisbecker <fweisbec@gmail.com>
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Date: Thu, 16 Apr 2009 19:53:48 +0200
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On Thu, 2009-04-16 at 13:45 -0400, Steven Rostedt wrote:
> On Thu, 16 Apr 2009, Peter Zijlstra wrote:
> 
> > On Thu, 2009-04-16 at 12:15 -0400, Steven Rostedt wrote:
> > > plain text document attachment
> > > (0002-tracing-events-lockdep-move-tracepoints-within-recu.patch)
> > > From: Steven Rostedt <srostedt@redhat.com>
> > > 
> > > With the current location of the tracepoints in lockdep, the system
> > > can hard lockup in minutes when the tracepoints are enabled.
> > > 
> > > Moving the tracepoints outside inside the lockdep protection solves
> > > the issue.
> > 
> > NAK
> 
> Can we at least add this as a workaround. Basically, the lockdep
> tracepoints are broken as is. I do not plan on changing the logic of the 
> events to prevent nesting. That's a feature I use. If we don't allow 
> nesting, we must drop events, which is bad.

No, I would say any nesting is a bug, and should stop tracing and print
a splat. No need to drop events.

> The other answer is simply to remove the trace points in lockdep, until 
> they work again.

Well, they worked when I added them, so someone broke it.