From: Christophe Leroy <christophe.leroy@csgroup.eu>
To: Segher Boessenkool <segher@kernel.crashing.org>,
Nick Desaulniers <ndesaulniers@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
Marco Elver <elver@google.com>,
Catalin Marinas <catalin.marinas@arm.com>,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
LKML <linux-kernel@vger.kernel.org>,
kasan-dev <kasan-dev@googlegroups.com>,
Mark Brown <broonie@kernel.org>,
Paul Mackerras <paulus@samba.org>,
linux-toolchains@vger.kernel.org, Will Deacon <will@kernel.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: Fri, 5 Mar 2021 07:38:25 +0100 [thread overview]
Message-ID: <ed3c08d2-04ba-217e-9924-28cab7750234@csgroup.eu> (raw)
In-Reply-To: <20210304192447.GT29191@gate.crashing.org>
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@google.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
next prev parent reply other threads:[~2021-03-05 6:38 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <e2e8728c4c4553bbac75a64b148e402183699c0c.1614780567.git.christophe.leroy@csgroup.eu>
[not found] ` <CANpmjNOvgbUCf0QBs1J-mO0yEPuzcTMm7aS1JpPB-17_LabNHw@mail.gmail.com>
[not found] ` <1802be3e-dc1a-52e0-1754-a40f0ea39658@csgroup.eu>
[not found] ` <YD+o5QkCZN97mH8/@elver.google.com>
[not found] ` <20210304145730.GC54534@C02TD0UTHF1T.local>
[not found] ` <CANpmjNOSpFbbDaH9hNucXrpzG=HpsoQpk5w-24x8sU_G-6cz0Q@mail.gmail.com>
[not found] ` <20210304165923.GA60457@C02TD0UTHF1T.local>
2021-03-04 17:25 ` [PATCH v1] powerpc: Include running function as first entry in save_stack_trace() and friends Marco Elver
2021-03-04 17:54 ` Nick Desaulniers
2021-03-04 19:24 ` Segher Boessenkool
2021-03-05 6:38 ` Christophe Leroy [this message]
2021-03-05 18:16 ` Segher Boessenkool
2021-03-04 18:01 ` Mark Rutland
2021-03-04 18:22 ` Marco Elver
2021-03-04 18:51 ` Mark Rutland
2021-03-04 19:01 ` Marco Elver
2021-03-05 12:04 ` Mark Rutland
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