linux-kernel - Re: [PATCH 2/3] trace: fix default boot up tracer

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Message-Id: <20090202210042.fdd589ff.akpm@linux-foundation.org>
Date:	Mon, 2 Feb 2009 21:00:42 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 2/3] trace: fix default boot up tracer

On Mon, 2 Feb 2009 23:33:52 -0500 (EST) Steven Rostedt <rostedt@...dmis.org> wrote:

> On Mon, 2 Feb 2009, Andrew Morton wrote:
> > > 
> > > The lock_kernel addition was added when the BKL became a spinlock again. 
> > > The selftests needed to be able to sleep, and this caused issues.
> > 
> > Sleeping inside lock_kernel() is quite OK.  Confused.
> 
> I did not explain that quite well. I need to focus on the emails 
> that I write, and not do it half concentrating on code that I'm
> also writing :-/
> 
> The preempt tracer expects preemption enabled when the self test is 
> executed. Because the self test for preempt tracer is basically:
> 
> start_trace();
> preempt_disable();
> udelay(x);
> preempt_enable();
> stop_trace();
> 
> make sure we have a delay.
> 
> This failed, because lock_kernel now disables preemption. So that 
> preempt_disable() never triggers the trace, and the test sees that nothing 
> was recorded. This causes a failure to be flagged, and we disable the 
> preempt tracer.

OK.

It might be a bit cleaner to run all the selftests later, after
start_kernel() has done unlock_kernel().  That would make it even
harder to support modular tracers in the future though.

Perhaps the preempt tracer could itself do

	if (kernel_locked()) {
		kernel_was_locked = true;
		unlock_kernel();
	}
	...
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