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

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Date:   Tue, 22 Nov 2022 09:35:17 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...nel.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 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.

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

---
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index c21b7347a26d..0354be027eb0 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -712,6 +712,7 @@ void arch_cpu_idle_enter(void)
 void arch_cpu_idle_dead(void)
 {
 	play_dead();
+	asm(ASM_REACHABLE);
 }
 
 /*