linux-kernel - Re: [PATCH 2/2] tracing/events/lockdep: move tracepoints within recursive protection

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Date:	Thu, 16 Apr 2009 13:38:05 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 2/2] tracing/events/lockdep: move tracepoints within
 recursive protection


On Thu, 16 Apr 2009, Peter Zijlstra wrote:

> On Thu, 2009-04-16 at 13:03 -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@...hat.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
> > > 
> > > the idea is to eventually move lockdep on top of the tracepoints. The
> > > tracer should grow to be more robust and handle recursion itself.
> > > 
> > > Its likely a case of the tracer using a spinlock or mutex in the
> > > tracepoint code. When I did the tracepoints I converted one such to a
> > > raw_spinlock_t in the trace_print code.
> > 
> > Note, that the ring buffer and events are made to be recursive. That is, 
> > it allows one event to trace within another event.
> 
> But surely not in the same context. You could do a 4 level recursion
> protection like I did in perf-counter, not allowing recursion in:
> 
>  nmi, irq, softirq, process - context.

Why not allow a nested interrupt to trace?

I don't want to add this logic to the lower levels, where only a few
users need the protection. The protecting should be at the user level.

> 
> That allows you to trace an irq while you're tracing something in
> process context, etc.. But not allow recursion on the same level.
> 
> >  If the tracepoint is 
> > triggered by something within the trace point handler, then we are 
> > screwed. That needs to be fixed.
> 
> Exactly the thing you want to detect and warn about, preferably with a
> nice stack trace.

Its hard when you want to allow nesting.

> 
> > I have not seen what is triggering back into locking. The ring buffer and 
> > what I can see by the event code, does not grab any locks besides raw 
> > ones.
> 
> Well, it used to all work, so something snuck in.

Note, it seems only the lockdep has issues with nesting. Perhaps when I 
can publish the lockless ring buffer this will all go away?

-- Steve

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