linux-kernel - Re: ftrace: preemptoff selftest not working

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Date:	Tue, 18 Nov 2008 14:48:35 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: ftrace: preemptoff selftest not working


* Heiko Carstens <heiko.carstens@...ibm.com> wrote:

> Hi Steven,
> 
> I was wondering why the preemptoff and preemptirqsoff tracer 
> selftests don't work on s390. After all its just that they get 
> called from non-preemptible context:
> 
> kernel_init() will execute all initcalls, however the first line in 
> kernel_init() is lock_kernel(), which causes the preempt_count to be 
> increased. Any later calls to add_preempt_count() (especially those 
> from the selftests) will therefore not result in a call to 
> trace_preempt_off() since the check below in add_preempt_count() 
> will be false:
> 
> 	if (preempt_count() == val)
> 		trace_preempt_off(CALLER_ADDR0, get_parent_ip(CALLER_ADDR1));
> 
> Hence the trace buffer will be empty.

ah, indeed :-)

side-effect of the removal of CONFIG_PREEMPT_BKL - these things were 
written before that and nobody noticed that the self-test stopped 
working for real.

> The patch below makes the selftests working for me, since then they 
> run in preemptible context. But it is ugly and I'm not proposing it 
> for upstream ;)
> 
> Just wanted to make you aware that there is a bug.

indeed it's ugly. We could perhaps drop the BKL in the selftests?

	Ingo
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