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Date:   Wed, 23 Nov 2022 10:19:41 -0800
From:   Josh Poimboeuf <jpoimboe@kernel.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>, sfr@canb.auug.org.au,
        linux-kernel@vger.kernel.org
Subject: Re: objtool warning for next-20221118
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On Wed, Nov 23, 2022 at 09:49:51AM -0800, Paul E. McKenney wrote:
> > > > Perhaps the best way would be to stick a REACHABLE annotation in
> > > > arch_cpu_idle_dead() or something?
> > > 
> > > When I apply this on -next, I still get the objtool complaint.
> > > Is there something else I should also be doing?
> > 
> > Silly GCC is folding the inline asm.  This works (but still doesn't seem
> > like the right approach):
> > 
> > diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> > index 26e8f57c75ad..128e7d78fedf 100644
> > --- a/arch/x86/kernel/process.c
> > +++ b/arch/x86/kernel/process.c
> > @@ -702,7 +702,7 @@ static void (*x86_idle)(void);
> >  #ifndef CONFIG_SMP
> >  static inline void play_dead(void)
> >  {
> > -	BUG();
> > +	_BUG_FLAGS(ASM_UD2, 0, ASM_REACHABLE);
> >  }
> >  #endif
> 
> I tried this, and still get:
> 
> vmlinux.o: warning: objtool: do_idle+0x156: unreachable instruction
> 
> Maybe my gcc is haunted?

Weird, it worked for me.  I have

  gcc version 12.2.1 20220819 (Red Hat 12.2.1-2) (GCC)

and I can't really fathom why that wouldn't work.  Maybe it's a
different issue?  The "unreachable instruction" warning is limited to
one, so when a first warning gets fixed, a second warning might suddenly
become visible.

Can you attach arch/x86/kernel/process.o?

-- 
Josh