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

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Date:   Thu, 1 Dec 2022 16:27:29 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Juergen Gross <jgross@...e.com>
Cc:     Josh Poimboeuf <jpoimboe@...nel.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "sfr@...b.auug.org.au" <sfr@...b.auug.org.au>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "sstabellini@...nel.org" <sstabellini@...nel.org>,
        "boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: objtool warning for next-20221118

On Tue, Nov 29, 2022 at 11:56:55AM -0800, Paul E. McKenney wrote:
> On Fri, Nov 25, 2022 at 06:30:35AM +0100, Juergen Gross wrote:
> > On 24.11.22 17:39, Josh Poimboeuf wrote:
> > > On Thu, Nov 24, 2022 at 08:47:47AM +0100, Juergen Gross wrote:
> > > > > > +++ b/arch/x86/xen/smp_pv.c
> > > > > > @@ -385,17 +385,9 @@ static void xen_pv_play_dead(void) /* used only
> > > > > > with HOTPLUG_CPU */
> > > > > >    {
> > > > > >        play_dead_common();
> > > > > >        HYPERVISOR_vcpu_op(VCPUOP_down, xen_vcpu_nr(smp_processor_id()), NULL);
> > > > > > -    cpu_bringup();
> > > > > > -    /*
> > > > > > -     * commit 4b0c0f294 (tick: Cleanup NOHZ per cpu data on cpu down)
> > > > > > -     * clears certain data that the cpu_idle loop (which called us
> > > > > > -     * and that we return from) expects. The only way to get that
> > > > > > -     * data back is to call:
> > > > > > -     */
> > > > > > -    tick_nohz_idle_enter();
> > > > > > -    tick_nohz_idle_stop_tick_protected();
> > > > > > -    cpuhp_online_idle(CPUHP_AP_ONLINE_IDLE);
> > > > > > +    /* FIXME: converge cpu_bringup_and_idle() and start_secondary() */
> > > > > > +    cpu_bringup_and_idle();
> > > > > 
> > > > > I think this will leak stack memory. Multiple cpu offline/online cycles of
> > > > > the same cpu will finally exhaust the idle stack.
> > > 
> > > Doh!  Of course...
> > > 
> > > I was actually thinking ahead, to where eventually xen_pv_play_dead()
> > > can call start_cpu0(), which can be changed to automatically reset the
> > > stack pointer like this:
> > > 
> > > SYM_CODE_START(start_cpu0)
> > > 	ANNOTATE_NOENDBR
> > > 	UNWIND_HINT_EMPTY
> > > 	movq	PER_CPU_VAR(pcpu_hot + X86_top_of_stack), %rax
> > > 	leaq	-PTREGS_SIZE(%rax), %rsp
> > > 	jmp	.Ljump_to_C_code
> > > SYM_CODE_END(start_cpu0)
> > > 
> > > but that would only be possible be after more cleanups which converge
> > > cpu_bringup_and_idle() with start_secondary().
> > > 
> > > > The attached patch seems to work fine.
> > > 
> > > The patch looks good to me.
> > > 
> > > It doesn't solve Paul's original issue where arch_cpu_idle_dead() needs
> > > to be __noreturn.  But that should probably be a separate patch anyway.
> > 
> > Okay, I'll split this off.
> > 
> > > 
> > > > The __noreturn annotation seems to trigger an objtool warning, though, in
> > > > spite of the added BUG() at the end of xen_pv_play_dead():
> > > > 
> > > > arch/x86/xen/smp_pv.o: warning: objtool: xen_pv_play_dead() falls through to
> > > > next function xen_pv_cpu_die()
> > > 
> > > You'll need to tell objtool that xen_cpu_bringup_again() is noreturn by
> > > adding "xen_cpu_bringup_again" to global_noreturns[] in
> > > tools/objtool/check.c.
> > 
> > Ah, okay. Will do that.
> > 
> > > (Yes it's a pain, I'll be working an improved solution to the noreturn
> > > thing...)
> > 
> > Should be fairly easy, no?
> > 
> > "Just" extend the __noreturn macro to put the function into a ".text.noreturn"
> > section, which can be handled in a special way by objtool. This would need
> > an __init_noreturn macro, of course, for a ".init.text.noreturn" section.
> 
> And in last night's -next run, that diagnostic was gone!
> 
> But of course another appeared in its place:
> 
> drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool: check_relocations+0xd1: stack state mismatch: cfa1=4+32 cfa2=-1+0
> 
> The .config file is shown below.  Thoughts?

And it is back.  Which makes no sense, but there it is.

							Thanx, Paul