Re: [PATCH] objtool: prefer memory clobber & %= to volatile & __COUNTER__

From: Nick Desaulniers <ndesaulniers@google.com>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>,
	Borislav Petkov <bp@alien8.de>, Vasily Gorbik <gor@linux.ibm.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Ingo Molnar <mingo@kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
	x86@kernel.org, llvm@lists.linux.dev,
	linux-sparse@vger.kernel.org, linux-kernel@vger.kernel.org,
	kernel test robot <lkp@intel.com>,
	Nathan Chancellor <nathan@kernel.org>,
	linux-toolchains@vger.kernel.org
Subject: Re: [PATCH] objtool: prefer memory clobber & %= to volatile & __COUNTER__
Date: Tue, 25 Jan 2022 16:59:12 -0800	[thread overview]
Message-ID: <CAKwvOd=P0VAFrrUXV0z5dES9hYP2b8-KwTzyG5=ezeUve=K3QA@mail.gmail.com> (raw)
In-Reply-To: <20220125233128.GT614@gate.crashing.org>

On Tue, Jan 25, 2022 at 3:34 PM Segher Boessenkool
<segher@kernel.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
