linux-kernel - Re: [PATCH v2] docs/arch: remove deprecated function name

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Message-ID: <e085496c-f267-44e7-94e3-c09521981439@infradead.org>
Date: Wed, 5 Feb 2025 16:33:02 -0800
From: Randy Dunlap <rdunlap@...radead.org>
To: Jiayuan Chen <mrpre@....com>, linux-doc@...r.kernel.org, corbet@....net
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] docs/arch: remove deprecated function name

Hi:

On 1/16/25 8:20 AM, Jiayuan Chen wrote:
> The dumpstack.c file has undergone many modifications, and the
> print_context_stack() function was removed or rewritten a long time ago,
> so it's better to remove the incorrect guidance.
> 
> I also want to preserve the original contributor info by keeping email
> adress and name.
> 
> Signed-off-by: Jiayuan Chen <mrpre@....com>
> ---
>  Documentation/arch/x86/kernel-stacks.rst | 51 +++++++++---------------
>  1 file changed, 18 insertions(+), 33 deletions(-)
> 
> diff --git a/Documentation/arch/x86/kernel-stacks.rst b/Documentation/arch/x86/kernel-stacks.rst
> index 738671a4070b..f780f4b09761 100644
> --- a/Documentation/arch/x86/kernel-stacks.rst
> +++ b/Documentation/arch/x86/kernel-stacks.rst
> @@ -112,41 +112,26 @@ Printing backtraces on x86
>  ==========================
>  
>  The question about the '?' preceding function names in an x86 stacktrace
> -keeps popping up, here's an indepth explanation. It helps if the reader
> -stares at print_context_stack() and the whole machinery in and around
> -arch/x86/kernel/dumpstack.c.
> +keeps popping up, here provides guidance about it. It helps if the reader

         popping up. This provides

> +stares at printk_stack_addressk() and its callers and pay special

                                                         pays

> +attention to the 'reliable' parameter ('?' basically means that the
> +address is unreliable).
>  
> -Adapted from Ingo's mail, Message-ID: <20150521101614.GA10889@...il.com>:
> +The detail about '?' can be found in the comments within dumpstack.c:
> +::
>  
> -We always scan the full kernel stack for return addresses stored on
> -the kernel stack(s) [1]_, from stack top to stack bottom, and print out
> -anything that 'looks like' a kernel text address.
> +    /*
> +     * Scan the stack, printing any text addresses we find.  At the
> +     * same time, follow proper stack frames with the unwinder.
> +     *
> +     * Addresses found during the scan which are not reported by
> +     * the unwinder are considered to be additional clues which are
> +     * sometimes useful for debugging and are prefixed with '?'.
> +     * This also serves as a failsafe option in case the unwinder
> +     * goes off in the weeds.
> +     */
>  
> -If it fits into the frame pointer chain, we print it without a question
> -mark, knowing that it's part of the real backtrace.
>  
> -If the address does not fit into our expected frame pointer chain we
> -still print it, but we print a '?'. It can mean two things:
> +You can also get more info from Ingo's original emal [1]_

                                                   email
and end that sentence with a period ('.').

>  
> - - either the address is not part of the call chain: it's just stale
> -   values on the kernel stack, from earlier function calls. This is
> -   the common case.
> -
> - - or it is part of the call chain, but the frame pointer was not set
> -   up properly within the function, so we don't recognize it.
> -
> -This way we will always print out the real call chain (plus a few more
> -entries), regardless of whether the frame pointer was set up correctly
> -or not - but in most cases we'll get the call chain right as well. The
> -entries printed are strictly in stack order, so you can deduce more
> -information from that as well.
> -
> -The most important property of this method is that we _never_ lose
> -information: we always strive to print _all_ addresses on the stack(s)
> -that look like kernel text addresses, so if debug information is wrong,
> -we still print out the real call chain as well - just with more question
> -marks than ideal.
> -
> -.. [1] For things like IRQ and IST stacks, we also scan those stacks, in
> -       the right order, and try to cross from one stack into another
> -       reconstructing the call chain. This works most of the time.
> +.. [1] https://lore.kernel.org/lkml/20150521101614.GA10889@gmail.com/

-- 
~Randy