Messages in this thread Patch in this message | | | Date | Tue, 22 Nov 2022 17:48:12 -0800 | From | Josh Poimboeuf <> | Subject | Re: objtool warning for next-20221118 |
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On Tue, Nov 22, 2022 at 04:22:58PM -0800, Paul E. McKenney wrote: > 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. > > > > 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
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