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Date: Thu, 14 Mar 2024 13:17:04 +0000
From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Sudeep Holla <sudeep.holla@arm.com>,
	"Christoph Lameter (Ampere)" <cl@linux.com>,
	Mark Rutland <mark.rutland@arm.com>,
	"linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Viresh Kumar <vireshk@kernel.org>, Will Deacon <will@kernel.org>,
	Jonathan.Cameron@huawei.com, Matteo.Carlini@arm.com,
	Valentin.Schneider@arm.com, akpm@linux-foundation.org,
	anshuman.khandual@arm.com, Eric Mackay <eric.mackay@oracle.com>,
	dave.kleikamp@oracle.com, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	robin.murphy@arm.com, vanshikonda@os.amperecomputing.com,
	yang@os.amperecomputing.com, Nishanth Menon <nm@ti.com>,
	Stephen Boyd <sboyd@kernel.org>
Subject: Re: [PATCH v3] ARM64: Dynamically allocate cpumasks and increase
 supported CPUs to 512
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Sender: Russell King (Oracle) <linux@armlinux.org.uk>

On Thu, Mar 14, 2024 at 01:28:40PM +0100, Marek Szyprowski wrote:
> Dear All,
> 
> On 14.03.2024 09:39, Catalin Marinas wrote:
> > On Wed, Mar 13, 2024 at 05:13:33PM +0000, Russell King wrote:
> >> So, I wonder whether what you're seeing is a latent bug which is
> >> being tickled by the presence of the CPU masks being off-stack
> >> changing the kernel timing.
> >>
> >> I would suggest the printk debug approach may help here to see when
> >> the OPPs are begun to be parsed, when they're created etc and their
> >> timing relationship to being used. Given the suspicion, it's possible
> >> that the mere addition of printk() may "fix" the problem, which again
> >> would be another semi-useful data point.
> > It might be an init order problem. Passing "initcall_debug" on the
> > cmdline might help a bit.
> >
> > It would also be useful in dev_pm_opp_set_config(), in the WARN_ON
> > block, to print opp_table->opp_list.next to get an idea whether it looks
> > like a valid pointer or memory corruption.
> 
> I've finally found some time to do the step-by-step printk-based 
> debugging of this issue and finally found what's broken!
> 
> Here is the fix:
> 
> diff --git a/drivers/cpufreq/cpufreq-dt.c b/drivers/cpufreq/cpufreq-dt.c
> index 8bd6e5e8f121..2d83bbc65dd0 100644
> --- a/drivers/cpufreq/cpufreq-dt.c
> +++ b/drivers/cpufreq/cpufreq-dt.c
> @@ -208,7 +208,7 @@ static int dt_cpufreq_early_init(struct device *dev, 
> int cpu)
>  ??????? if (!priv)
>  ??????????????? return -ENOMEM;
> 
> -?????? if (!alloc_cpumask_var(&priv->cpus, GFP_KERNEL))
> +?????? if (!zalloc_cpumask_var(&priv->cpus, GFP_KERNEL))
>  ??????????????? return -ENOMEM;
> 
>  ??????? cpumask_set_cpu(cpu, priv->cpus);
> 
> 
> It is really surprising that this didn't blow up for anyone else so 
> far... This means that the $subject patch is fine.

Wow. I guess we've been lucky with that allocation hitting memory
containing zeros. Well done at tracking it down!

-- 
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