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

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Message-ID: <alpine.DEB.1.10.0811180931270.15003@gandalf.stny.rr.com>
Date:	Tue, 18 Nov 2008 09:32:26 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux-kernel@...r.kernel.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: ftrace: preemptoff selftest not working


On Tue, 18 Nov 2008, Ingo Molnar wrote:
> 
> * 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)

I have no problem with that, but does the BKL play any role for being 
held? I have no idea why it is taken in boot up, so I'm hestiant to touch 
it.

-- Steve

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