Messages in this thread | | | Date | Wed, 23 Nov 2022 09:49:51 -0800 | From | "Paul E. McKenney" <> | Subject | Re: objtool warning for next-20221118 |
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On Tue, Nov 22, 2022 at 05:48:12PM -0800, Josh Poimboeuf wrote: > 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
I tried this, and still get:
vmlinux.o: warning: objtool: do_idle+0x156: unreachable instruction
Maybe my gcc is haunted?
Thanx, Paul
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