linux-kernel - Re: [RFC PATCH v1 1/3] Revert "powerpc/bug: Provide better flexibility to WARN_ON/__WARN

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Date:   Tue, 20 Jun 2023 09:10:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Naveen N Rao <naveen@...nel.org>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Sathvika Vasireddy <sv@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Subject: Re: [RFC PATCH v1 1/3] Revert "powerpc/bug: Provide better
 flexibility to WARN_ON/__WARN_FLAGS() with asm goto"

On Tue, Jun 20, 2023 at 10:51:25AM +0530, Naveen N Rao wrote:
> Christophe Leroy wrote:
> > This reverts commit 1e688dd2a3d6759d416616ff07afc4bb836c4213.
> > 
> > That commit aimed at optimising the code around generation of
> > WARN_ON/BUG_ON but this leads to a lot of dead code erroneously
> > generated by GCC.
> > 
> >      text	   data	    bss	    dec	    hex	filename
> >   9551585	3627834	 224376	13403795	 cc8693	vmlinux.before
> >   9535281	3628358	 224376	13388015	 cc48ef	vmlinux.after
> > 
> > Once this change is reverted, in a standard configuration (pmac32 +
> > function tracer) the text is reduced by 16k which is around 1.7%
> 
> Aneesh recently reported a build failure due to the use of 'asm goto' in
> WARN_ON(). We were able to root-cause it to the use of 'asm goto' with two
> config options: CONFIG_CC_OPTIMIZE_FOR_SIZE and
> CONFIG_DEBUG_SECTION_MISMATCH.

FWIW;

I recently had clang-powerpc report a very dodgy build error that was
due to a combination of these asm-goto and the usage of __cleanup__.
For some reason the label of the asm-goto crossed over the __cleanup__
variable declaration -- which is not valid, but also was completely
insane for that's not what the code called for.

  https://lkml.kernel.org/r/20230610082005.GB1370249@hirez.programming.kicks-ass.net

But in my book that's a compiler issue, not a kernel issue and I'd be
hesitant to pull the asm-goto use just for that.