From: "Paul E. McKenney" <paulmck@kernel.org>
To: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
sfr@canb.auug.org.au, linux-kernel@vger.kernel.org
Subject: Re: objtool warning for next-20221118
Date: Wed, 23 Nov 2022 09:49:51 -0800 [thread overview]
Message-ID: <20221123174951.GZ4001@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <20221123014812.7gptbvvndzemt5nc@treble>
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
next prev parent reply other threads:[~2022-11-23 17:50 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-21 4:07 objtool warning for next-20221118 Paul E. McKenney
2022-11-21 11:31 ` Peter Zijlstra
2022-11-21 14:52 ` Paul E. McKenney
2022-11-22 5:16 ` Josh Poimboeuf
2022-11-22 8:35 ` Peter Zijlstra
2022-11-23 0:22 ` Paul E. McKenney
2022-11-23 1:48 ` Josh Poimboeuf
2022-11-23 17:49 ` Paul E. McKenney [this message]
2022-11-23 18:19 ` Josh Poimboeuf
2022-11-23 19:12 ` Paul E. McKenney
2022-11-23 22:32 ` Josh Poimboeuf
2022-11-23 23:06 ` Paul E. McKenney
2023-01-28 19:06 ` Paul E. McKenney
2023-02-01 0:02 ` Josh Poimboeuf
2023-02-01 0:33 ` Paul E. McKenney
2023-02-01 4:21 ` Paul E. McKenney
2022-11-23 1:23 ` Josh Poimboeuf
2022-11-23 8:55 ` Peter Zijlstra
2022-11-23 10:52 ` Andrew Cooper
2022-11-23 17:03 ` Josh Poimboeuf
2022-11-24 2:39 ` Josh Poimboeuf
2022-11-24 5:28 ` Juergen Gross
2022-11-24 7:47 ` Juergen Gross
2022-11-24 16:39 ` Josh Poimboeuf
2022-11-25 5:30 ` Juergen Gross
2022-11-29 19:56 ` Paul E. McKenney
2022-12-02 0:27 ` Paul E. McKenney
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20221123174951.GZ4001@paulmck-ThinkPad-P17-Gen-1 \
--to=paulmck@kernel.org \
--cc=jpoimboe@kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=peterz@infradead.org \
--cc=sfr@canb.auug.org.au \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).