linux-kernel - Re: [PATCH 6/7] smp: Don't yell about IRQs disabled in kgdb_roundup

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Date:   Tue, 30 Oct 2018 09:25:25 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Jason Wessel <jason.wessel@...driver.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        tglx@...utronix.de, mingo@...nel.org, gregkh@...uxfoundation.org,
        linux-arm-msm@...r.kernel.org,
        kgdb-bugreport@...ts.sourceforge.net, linux-mips@...ux-mips.org,
        linux-sh@...r.kernel.org, linux-hexagon@...r.kernel.org,
        frederic@...nel.org, riel@...riel.com,
        linux-kernel@...r.kernel.org, luto@...nel.org,
        sparclinux@...r.kernel.org, linux-snps-arc@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 6/7] smp: Don't yell about IRQs disabled in
 kgdb_roundup_cpus()

On Mon, Oct 29, 2018 at 11:07:06AM -0700, Douglas Anderson wrote:
> In kgdb_roundup_cpus() we've got code that looks like:
>   local_irq_enable();
>   smp_call_function(kgdb_call_nmi_hook, NULL, 0);
>   local_irq_disable();

> Let's add kgdb to the list of reasons not to warn in
> smp_call_function_many().  That will allow us (in a future patch) to
> stop calling local_irq_enable() which will get rid of the original
> splat.
> 
> NOTE: with this change comes the obvious question: will we start
> deadlocking more often now when we drop into the debugger.  I can't
> say that for sure one way or the other, but the fact that we do the
> same logic for "oops_in_progress" makes me feel like it shouldn't
> happen too often.  Also note that the old logic of turning on
> interrupts temporarily wasn't exactly safe since (I presume) that
> could have violated spin_lock_irqsave() semantics and ended up with a
> deadlock of its own.

How is any of that not utterly and terminally broken?

> @@ -413,7 +414,8 @@ void smp_call_function_many(const struct cpumask *mask,
>  	 * can't happen.
>  	 */
>  	WARN_ON_ONCE(cpu_online(this_cpu) && irqs_disabled()
> -		     && !oops_in_progress && !early_boot_irqs_disabled);
> +		     && !oops_in_progress && !early_boot_irqs_disabled
> +		     && !in_dbg_master());
>  
>  	/* Try to fastpath.  So, what's a CPU they want? Ignoring this one. */
>  	cpu = cpumask_first_and(mask, cpu_online_mask);

Not a fan of this. There is a distinct difference between
oops_in_progress and dropping into kgdb in that you don't ever expect to
return/survive oopses, whereas we do expect to survive kgdb.

Also, how does kgdb work at all without actual NMIs ?

So no, NAK on this.