From mboxrd@z Thu Jan  1 00:00:00 1970
Date: Sat, 18 Jun 2016 15:52:35 +0200
From: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Message-ID: <20160618135235.GE18433@hermes.click-hack.org>
References: <20160618130822.GB18433@hermes.click-hack.org>
 <8e8bb173-eda9-bab4-6236-634da1e396cb@xenomai.org>
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Subject: Re: [Xenomai] ftrace dispatch function broken on 2.6.5
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To: Philippe Gerum <rpm@xenomai.org>
Cc: Xenomai <xenomai@xenomai.org>

On Sat, Jun 18, 2016 at 03:21:40PM +0200, Philippe Gerum wrote:
> On 06/18/2016 03:08 PM, Gilles Chanteperdrix wrote:
> > Hi Philippe,
> > 
> > it seems since I-pipe commit
> > b115c4094d734e19fa7a96be1bf3958b3d244b8b on the ipipe-3.18 branch:
> >     Revert "ipipe: Register function tracer for direct and exclusive invocation"
> >     
> >     This reverts commit e00888b4aae45d9b84698a62079dde14c9be5fd3.
> >     
> >     We now have an I-pipe-compatible dispatching function for ftrace.
> > 
> > The ftrace dispatching function causes the following warning at
> > boot on x86_32 with all warnings/debugs enabled:
> > [    4.730812] I-pipe: head domain Xenomai registered.
> > [    4.737967] I-pipe: Detected illicit call from head domain 'Xenomai'
> > [    4.737967]         into a regular Linux service
> > 
> > Because it calls preempt_disable(), which is not safe to be called
> > form root domain, when runnning over 2.6.x on an architecture such
> > as x86_32 which does not have IPIPE_HAVE_SAFE_THREAD_INFO.
> > 
> > Should we make the ftrace dispatching function really I-pipe
> > compatible by calling ipipe_preempt_disable() in that case instead?
> > or should we make the patch revert conditional to !IPIPE_LEGACY or
> > IPIPE_HAVE_SAFE_THREAD_INFO (but that would make only the I-pipe
> > tracer work in that case).
> > 
> 
> I would go for the change which has the lesser impact on the mainline
> code; that would be option #1.

Ok, now I get:

   12.958818] I-pipe: Detected stalled head domain, probably caused by a bug.
[   12.958818]         A critical section may have been left unterminated.

I guess in the following sequence:

#define hard_preempt_disable()				\
	({						\
		unsigned long __flags__;		\
		__flags__ = hard_local_irq_save();	\
		if (__ipipe_root_p)			\
			preempt_disable();		\
		__flags__;				\
	})

preempt_disable() gets called with the head domain stalled, while
current domain is root, and ipipe_root_only() treats this as an
error.

-- 
					    Gilles.
https://click-hack.org