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

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Message-ID: <20081118135221.GE31146@elte.hu>
Date:	Tue, 18 Nov 2008 14:52:21 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux-kernel@...r.kernel.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: ftrace: preemptoff selftest not working


* Steven Rostedt <rostedt@...dmis.org> wrote:

> > Hence the trace buffer will be empty. 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.
> 
> Yep, this might be a better answer than what I put into linux-tip 
> (and my git repo).
> 
> See:
> 
>   ftrace: force pass of preemptoff selftest
> 
> The cause of the bug was the conversion of the BKL back to a 
> spinlock, and making it non preempt.  The initcall code is called 
> with the BKL applied which now means it can not preempt. This breaks 
> the preempt tracer selftest.
> 
> My solution was to just force a pass if this is detected. Perhaps 
> moving the test might be better.

it would be better to just drop the BKL in that selftest. (or in all 
selftests - an elevated preempt count will skew a number of things)

	Ingo
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