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Date: Wed, 22 Apr 2015 11:21:34 -0400
From: Don Zickus <dzickus@redhat.com>
To: Ulrich Obergfell <uobergfe@redhat.com>
Cc: Chris Metcalf <cmetcalf@ezchip.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
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        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v9 2/3] watchdog: add watchdog_cpumask sysctl to assist
 nohz
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On Wed, Apr 22, 2015 at 07:02:31AM -0400, Ulrich Obergfell wrote:
> 
> Chris,
> 
> in https://lkml.org/lkml/2015/4/17/616 you stated:
> 
>  ">> +	alloc_cpumask_var(&watchdog_cpumask_for_smpboot, GFP_KERNEL);
>   >
>   > alloc_cpumask_var could fail?
> 
>   Good catch; if I get a failure I'll just return early without trying to
>   start the watchdog, since clearly things are too memory-constrained
>   to enable that functionality anyway."
> 
> Let's assume that (in spite of the memory constraints) the kernel would still
> be able to make progress and get to a point where the system will be usable.
> In this corner case, the following code would leave a NULL pointer behind in
> watchdog_cpumask and in watchdog_cpumask_bits which could subsequently lead
> to a crash.
> 
>  void __init lockup_detector_init(void)
>  {
>          set_sample_period();
>  
> +        if (!alloc_cpumask_var(&watchdog_cpumask, GFP_KERNEL)) {
> +                pr_err("Failed to allocate cpumask for watchdog");
> +                return;
> +        }
> +        watchdog_cpumask_bits = cpumask_bits(watchdog_cpumask);
> 
> For example, proc_watchdog_cpumask() and the change that your patch introduces
> in watchdog_enable_all_cpus() are not protected against a possible NULL pointer.
> I think the code needs to be made safer.

Or we could just statically allocate it

static DECLARE_BITMAP(watchdog_cpumask, NR_CPUS) __read_mostly;

Cheers,
Don