linux-kernel - Re: [GIT pull] smp/hotplug updates for 4.13

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Date:   Tue, 11 Jul 2017 08:09:16 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [GIT pull] smp/hotplug updates for 4.13

On Sun, 9 Jul 2017, Linus Torvalds wrote:
> On Sun, Jul 9, 2017 at 2:07 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> >
> > +       /* Wait for the CPU to reach CPUHP_AP_ONLINE_IDLE */
> >         wait_for_completion(&st->done);
> > +       BUG_ON(!cpu_online(cpu));
> 
> I realize this isn't actually a new BUG_ON(), just a moved one, but
> can we *please* just agree to get rid of these machine-killing
> idiocies?
> 
> The fact that it would be a bug if this triggers doesn't excuse it one
> whit. BUG_ON() is simply bad.
> 
> Either it's a "this cannot happen", in which case it should be
> deleted, or it's a "if this ever happens, we really should notice it",
> in which case it should probably be a
> 
>    if (WARN_ON)ONCE(..))
>       return -EBUSY;
> 
> or something.
> 
> That routine very much has an existing error path for failure to bring
> up a CPU, dammit!
> 
> THERE IS NO EXCUSE FOR KILLING THE MACHINE THERE!

Agreed. I noticed when I moved the code, but then forgot. Will queue a fix.

Thanks,

	tglx