linux-kernel - Re: [Linux 2.6.29-rc2] BUG: using smp_processor

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Date:	Mon, 26 Jan 2009 22:35:19 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Frédéric Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Maciej Rutecki <maciej.rutecki@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Linux 2.6.29-rc2] BUG: using smp_processor_id() in preemptible


* Rafael J. Wysocki <rjw@...k.pl> wrote:

> On Monday 26 January 2009, Ingo Molnar wrote:
[...]
> > It would work like this, you could mark IRQs as disabled 'permanently':
> > 
> >    force_irqs_off_start();
> >    ...
> >    force_irqs_off_end();
> > 
> > you could mark an arbitrarily complex code sequence that way, and ftrace 
> > would emit a WARN_ONCE() if irqs are enable anytime during that sequence - 
> > by using the irq-tracking facilities we have for the irqsoff tracer (and 
> > which we also have for lockdep).
> > 
> > Would that be useful?
> 
> Not sure, I only know a little about ftrace, I really can't judge.

The instrumentation is really simple, see kernel/tracing/trace_irqsoff.c:

we call this function if hardirqs are disabled anywhere in the kernel:

 static inline void
 start_critical_timing(unsigned long ip, unsigned long parent_ip)

and we call this function if hardirqs are enabled anywhere in the kernel:

 static inline void
 stop_critical_timing(unsigned long ip, unsigned long parent_ip)

that's all. You need a single line check in stop_critical_timing(), 
something like this:

   WARN_ON_ONCE(per_cpu(hardirqs_forced_off, this_cpu));

> Anyway, I think that putting the checks directly into the code path in 
> question would be more reliable and would still work without ftrace.

More reliable than a WARN() triggering right at the buggy place that 
erroneously enables IRQs? Regardless of how obscurely it's done - whether 
it's a side effect of something, etc. etc.?

With such a generic facility you'd not have to put in any explicit checks 
anywhere _at all_.

In fact whatever check you put in it's _always_ going to be fundamentally 
more fragile than direct instrumentation: you cannot possibly check all 
possible places that enable interrupts. (they could be disabling 
interrupts as a _restore_irqs() sequence for example)

	Ingo
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