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

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Date:   Tue, 18 Jan 2022 15:01:20 -0800
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        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>
Subject: Re: [PATCH] objtool: prefer memory clobber & %= to volatile &
 __COUNTER__

On Tue, Jan 18, 2022 at 11:22:59AM -0800, Josh Poimboeuf wrote:
> On Sun, Jan 16, 2022 at 02:32:59PM +0100, Borislav Petkov wrote:
> > so I like the local label and how it is more readable this way.
> > 
> > So, provided the memory clobber works (I wonder here if Josh has some
> > concrete failing cases which could be tested with your version) and
> > after the nitpicks have been addressed
> > 
> > Acked-by: Borislav Petkov <bp@...e.de>
> 
> I think Nick was already able to recreate the original issue.  I'll run
> it through some more testing.
> 
> I wanted to make this change years ago, but couldn't because of legacy
> toolchains.  Here's hoping this is the final solution for those @#$%^
> macros.
> 
> Boris, thanks for looping Nick in, I should have done so to begin with.

Apparently this patch isn't going to work after all :-(

  https://lkml.kernel.org/r/202201190632.lhlaiCBk-lkp@intel.com

With the two WARN_ONs in media_request_object_complete(), GCC apparently
considers the two reachable() asm statements as duplicates, and it
removes the second one.

-- 
Josh