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Subject: Re: [RFC PATCH] Add TRACE_IRQFLAGS_SUPPORT, LOCKDEP_SUPPORT then 
 enable ftrace for ia64
From: Peter Zijlstra <peterz@infradead.org>
To: Tony Luck <tony.luck@intel.com>
Cc: Luming Yu <luming.yu@gmail.com>, Ingo Molnar <mingo@elte.hu>,
       Christoph Hellwig <hch@infradead.org>,
       LKML <linux-kernel@vger.kernel.org>,
       "linux-ia64@vger.kernel.org" <linux-ia64@vger.kernel.org>,
       "Yu, Fenghua" <fenghua.yu@intel.com>,
       "Li, Shaohua" <shaohua.li@intel.com>
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Date: Wed, 09 Dec 2009 19:22:52 +0100
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On Wed, 2009-12-09 at 10:11 -0800, Tony Luck wrote:
> On Wed, Dec 9, 2009 at 12:52 AM, Luming Yu <luming.yu@gmail.com> wrote:
> > Commenting out the change make -32 Lockdep kernel boot.
> >
> > Tony, please let me know whether you can reproduce..
> 
> Yes.  Commenting this out makes the system bootable for me.
> 
> > kernel/lockdep.c
> > static int save_trace(struct stack_trace *trace)
> > {
> >        trace->nr_entries = 0;
> >        trace->max_entries = MAX_STACK_TRACE_ENTRIES - nr_stack_trace_entries;
> >        trace->entries = stack_trace + nr_stack_trace_entries;
> >
> >        trace->skip = 3;
> >
> >        save_stack_trace(trace);
> > #if 0
> >
> >        /*
> >         * Some daft arches put -1 at the end to indicate its a full trace.
> >         *
> >         * <rant> this is buggy anyway, since it takes a whole extra entry so a
> >         * complete trace that maxes out the entries provided will be reported
> >         * as incomplete, friggin useless </rant>
> >         */
> >        if (trace->entries[trace->nr_entries-1] == ULONG_MAX)
> >                trace->nr_entries--;
> > #endif
> 
> This happens because ia64 save_stack_trace() is currently an empty stub, so it
> doesn't set trace->nr_entries.
> 
> Options:
> 1) Add trace->nr_entries = 1; to ia64 stub.
> 2) Change the generic code to defend against an unexpected failure in
> save_stack_trace()
> 
>        if (trace->nr_entries > 0 &&
>            trace->entries[trace->nr_entries-1] == ULONG_MAX)
>                trace->nr_entries--;
> 3) Delete the code in the "daft"[1] architectures, and drop this hunk
> from the generic code.

I'm thinking 2 and 3 together sound like a plan :-)

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