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

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Message-ID: <20210304192447.GT29191@gate.crashing.org>
Date:   Thu, 4 Mar 2021 13:24:47 -0600
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Marco Elver <elver@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>,
        Paul Mackerras <paulus@...ba.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        linux-toolchains@...r.kernel.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.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

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")

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.


Segher