linux-kernel - Re: objtool warning for next-20221118

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Date:   Tue, 22 Nov 2022 17:23:50 -0800
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>, sfr@...b.auug.org.au,
        linux-kernel@...r.kernel.org
Subject: Re: objtool warning for next-20221118

On Tue, Nov 22, 2022 at 09:35:17AM +0100, Peter Zijlstra wrote:
> On Mon, Nov 21, 2022 at 09:16:05PM -0800, Josh Poimboeuf wrote:
> 
> > It's complaining about an unreachable instruction after a call to
> > arch_cpu_idle_dead().  In this case objtool detects the fact
> > arch_cpu_idle_dead() doesn't return due to its call to the
> > non-CONFIG_SMP version of play_dead().  But GCC has no way of detecting
> > that because the caller is in another translation unit.
> > 
> > As far as I can tell, that function should never return.  Though it
> > seems to have some dubious semantics (see xen_pv_play_dead() for
> > example, which *does* seem to return?).  I'm thinking it would be an
> > improvement to enforce that noreturn behavior across all arches and
> > platforms, sprinkling __noreturn and BUG() on arch_cpu_idle_dead() and
> > maybe some of it callees, where needed.
> > 
> > Peter, what do you think?  I could attempt a patch.
> 
> I'm thinking the Xen case makes all this really rather difficult :/
> 
> While normally a CPU is brought up through a trampoline, Xen seems to
> have implemented it by simply returning from play_dead(), and afaict
> that is actually a valid way to go about doing it.

o_O

How the @#$% is that a valid way of doing it?  Why not just do it the
normal way?

> Perhaps the best way would be to stick a REACHABLE annotation in
> arch_cpu_idle_dead() or something?

It what universe would we expect a function named "play_dead" to instead
finish bringing the CPU up and return?

That's just awful...  I don't see anything "valid" about it.

-- 
Josh