linux-kernel - Re: [PATCH] ARM: Adjust backtrace messages to line up with x86 kernel

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Message-ID: <CAPnjgZ0jGZJ4O2=M-nymf5kT0mfvKRRcgb36E=UV_uznoYaroA@mail.gmail.com>
Date:	Tue, 17 Jan 2012 23:01:37 -0800
From:	Simon Glass <sjg@...omium.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Stephen Boyd <sboyd@...eaurora.org>
Subject: Re: [PATCH] ARM: Adjust backtrace messages to line up with x86 kernel

Hi Russell,

On Tue, Jan 17, 2012 at 12:24 PM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> On Tue, Jan 17, 2012 at 12:16:01PM -0800, Simon Glass wrote:
>> It is nice to print the backtrace message on its own line, and also
>> regardless of the setting of CONFIG_ARM_UNWIND. This means that tools
>> which parse the backtrace don't need a special case for the first line.
>> It also makes it more like the x86 kernel output.
>>
>> I believe we should also put KERN_EMERG at the start of this. Otherwise
>> it won't appear in console output (although it will appear in the logs
>> after a reboot).
>>
>> The impact of this change is that the log output changes from something
>> like:
>>
>> [    2.861692] 7fe0: 01beb1b8 befcb7d4 0000aebb 40284334 80000010 01be4668 00000000 00000000
>> [    2.869866] Backtrace: [<c00bb3f4>] (module_put+0x44/0xc0) from [<c00bdd98>] (sys_init_module+0x186c/0x19c4)
>
> You should not be getting it like this; I've never seen an oops dump
> containing stuff formatted in this way on ARM.
>
> If you care to look at the code, you'll notice that we have:
>
>        printk("Backtrace: ");
> ...
>        if () {
>                printk("no frame pointer");
>        } else if () {
>                printk("invalid frame pointer 0x%08x", fp);
>        } else if ()
>                printk("frame pointer underflow");
>        printk("\n");
>
>        if (ok)
>                c_backtrace();
>
> So, by adding the '\n' at the end of the first printk, not only will you
> add an additional blank line to the backtrace, but also move those
> backtrace warning messages onto their own line, further pushing up the
> number of lines an oops dump produces.
>
> However, the lack of "Backtrace:" with ARM_UNWIND set needs fixing -
> but please look at the unwind_backtrace() function first, and notice
> that it contains its own version of this message (using __func__
> instead - which is probably a bad idea.)  That needs fixing too.

Sorry my patch was so awful but I am pleased to get a response.

I will send a v2 - but keep the pr_debug() in unwind_backtrace() since
I don't see a reason to remove it. Because it is pr_debug() it
generally won't appear anyway I suppose.

Regards,
Simon
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