linux-kernel - Re: [PATCH] objtool: prefer memory clobber & %= to volatile & __COUNTER_

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Message-ID: <CAKwvOd=P0VAFrrUXV0z5dES9hYP2b8-KwTzyG5=ezeUve=K3QA@mail.gmail.com>
Date:   Tue, 25 Jan 2022 16:59:12 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        x86@...nel.org, llvm@...ts.linux.dev, linux-sparse@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel test robot <lkp@...el.com>,
        Nathan Chancellor <nathan@...nel.org>,
        linux-toolchains@...r.kernel.org
Subject: Re: [PATCH] objtool: prefer memory clobber & %= to volatile & __COUNTER__

On Tue, Jan 25, 2022 at 3:34 PM Segher Boessenkool
<segher@...nel.crashing.org> wrote:
>
> Hi!
>
> On Mon, Jan 24, 2022 at 03:26:36PM -0800, Nick Desaulniers wrote:
>
> > If this is fixed in gcc-10, then we can probably add a comment with a
> > FIXME link to the issue or commit to replace __COUNTER__ with %= one
> > day.  If not, then we can probably come up with a reduced test case
> > for the GCC devs to take a look at, then add the FIXME comment to
> > kernel sources.
>
> Please open a PR?

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104236

> > I'm more confident that we can remove the `volatile` keyword (I was
> > thinking about adding a new diagnostic to clang to warn that volatile
> > is redundate+implied for asm goto or inline asm that doesn't have
> > outputs) though that's not the problem here and will probably generate
> > some kernel wide cleanup before we could enable such a flag.
>
> Its main value is that it would discourage users from thinking volatile
> is magic.  Seriously worth some pain!

Yeah, SGTM.

>
> > Perhaps
> > there are known compiler versions that still require the keyword for
> > those cases for some reason.
>
> It was removed from compiler-gcc.h in 3347acc6fcd4 (which changed the
> minimum required GCC version to GCC 5).

```
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index e512f5505dad..b8fe0c23cfff 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -80,11 +80,25 @@ void ftrace_likely_update(struct
ftrace_likely_data *f, int val,

 /* Optimization barrier */
 #ifndef barrier
-# define barrier() __memory_barrier()
+/* The "volatile" is due to gcc bugs */
+# define barrier() __asm__ __volatile__("": : :"memory")
```

I definitely wish there was a comment with a link to what "gcc bugs"
they were referring to; otherwise who knows if it's been fixed...if
they have been...
-- 
Thanks,
~Nick Desaulniers