linux-kernel - Re: [PATCH v1] powerpc: Include running function as first entry in save_stack

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Message-ID: <ed3c08d2-04ba-217e-9924-28cab7750234@csgroup.eu>
Date:   Fri, 5 Mar 2021 07:38:25 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Segher Boessenkool <segher@...nel.crashing.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Marco Elver <elver@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Mark Brown <broonie@...nel.org>,
        Paul Mackerras <paulus@...ba.org>,
        linux-toolchains@...r.kernel.org, Will Deacon <will@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v1] powerpc: Include running function as first entry in
 save_stack_trace() and friends



Le 04/03/2021 à 20:24, Segher Boessenkool a écrit :
> On Thu, Mar 04, 2021 at 09:54:44AM -0800, Nick Desaulniers wrote:
>> On Thu, Mar 4, 2021 at 9:42 AM Marco Elver <elver@...gle.com> wrote:
>> include/linux/compiler.h:246:
>> prevent_tail_call_optimization
>>
>> commit a9a3ed1eff36 ("x86: Fix early boot crash on gcc-10, third try")

https://github.com/linuxppc/linux/commit/a9a3ed1eff36

> 
> That is much heavier than needed (an mb()).  You can just put an empty
> inline asm after a call before a return, and that call cannot be
> optimised to a sibling call: (the end of a function is an implicit
> return:)
> 
> Instead of:
> 
> void g(void);
> void f(int x)
> 	if (x)
> 		g();
> }
> 
> Do:
> 
> void g(void);
> void f(int x)
> 	if (x)
> 		g();
> 	asm("");
> }
> 
> This costs no extra instructions, and certainly not something as heavy
> as an mb()!  It works without the "if" as well, of course, but with it
> it is a more interesting example of a tail call.

In the commit mentionned at the top, it is said:

The next attempt to prevent compilers from tail-call optimizing
the last function call cpu_startup_entry(), ... , was to add an empty asm("").

This current solution was short and sweet, and reportedly, is supported
by both compilers but we didn't get very far this time: future (LTO?)
optimization passes could potentially eliminate this, which leads us
to the third attempt: having an actual memory barrier there which the
compiler cannot ignore or move around etc.

Christophe