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Date: Wed, 9 Dec 2009 10:11:20 -0800
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Message-ID: <12c511ca0912091011g1d6308c4if7684adf9b0c5baf@mail.gmail.com>
Subject: Re: [RFC PATCH] Add TRACE_IRQFLAGS_SUPPORT, LOCKDEP_SUPPORT then 
	enable ftrace for ia64
From: Tony Luck <tony.luck@intel.com>
To: Luming Yu <luming.yu@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>, Peter Zijlstra <a.p.zijlstra@chello.nl>,
       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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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.

-Tony

[1] arm, parisc, s390, sh, x86
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