Re: [PATCH v1] powerpc: Include running function as first entry in save_stack_trace() and friends

From: Segher Boessenkool <segher@kernel.crashing.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Marco Elver <elver@google.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Mark Brown <broonie@kernel.org>,
	Paul Mackerras <paulus@samba.org>,
	kasan-dev <kasan-dev@googlegroups.com>,
	linux-toolchains@vger.kernel.org,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v1] powerpc: Include running function as first entry in save_stack_trace() and friends
Date: Thu, 4 Mar 2021 13:24:47 -0600	[thread overview]
Message-ID: <20210304192447.GT29191@gate.crashing.org> (raw)
In-Reply-To: <CAKwvOd=wBArMwvtDC8zV-QjQa5UuwWoxksQ8j+hUCZzbEAn+Fw@mail.gmail.com>

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@google.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