linux-kernel - Re: [RFC PATCH 20/23] watchdog/hardlockup/hpet: Rotate interrupt among all monitored CPUs

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Date:   Wed, 13 Jun 2018 11:48:09 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
cc:     Ingo Molnar <mingo@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Andi Kleen <andi.kleen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, Borislav Petkov <bp@...e.de>,
        Tony Luck <tony.luck@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>, x86@...nel.org,
        sparclinux@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, Jacob Pan <jacob.jun.pan@...el.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Don Zickus <dzickus@...hat.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Frederic Weisbecker <frederic@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Babu Moger <babu.moger@...cle.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Colin Ian King <colin.king@...onical.com>,
        Byungchul Park <byungchul.park@....com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Christoffer Dall <cdall@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Kai-Heng Feng <kai.heng.feng@...onical.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        David Rientjes <rientjes@...gle.com>,
        iommu@...ts.linux-foundation.org
Subject: Re: [RFC PATCH 20/23] watchdog/hardlockup/hpet: Rotate interrupt
 among all monitored CPUs

On Tue, 12 Jun 2018, Ricardo Neri wrote:
> +	/* There are no CPUs to monitor. */
> +	if (!cpumask_weight(&hdata->monitored_mask))
> +		return NMI_HANDLED;
> +
>  	inspect_for_hardlockups(regs);
>  
> +	/*
> +	 * Target a new CPU. Keep trying until we find a monitored CPU. CPUs
> +	 * are addded and removed to this mask at cpu_up() and cpu_down(),
> +	 * respectively. Thus, the interrupt should be able to be moved to
> +	 * the next monitored CPU.
> +	 */
> +	spin_lock(&hld_data->lock);

Yuck. Taking a spinlock from NMI ... 

> +	for_each_cpu_wrap(cpu, &hdata->monitored_mask, smp_processor_id() + 1) {
> +		if (!irq_set_affinity(hld_data->irq, cpumask_of(cpu)))
> +			break;

... and then calling into generic interrupt code which will take even more
locks is completely broken.

Guess what happens when the NMI hits a section where one of those locks is
held? Then you need another watchdog to decode the lockup you just ran into.

Thanks,

	tglx