linux-kernel - Re: [PATCH v7 1/3] cacheinfo: Allocate memory during CPU hotplug if not done from the primary CPU

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Message-ID: <20241008170015.GA24312@ranerica-svr.sc.intel.com>
Date: Tue, 8 Oct 2024 10:00:15 -0700
From: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To: Borislav Petkov <bp@...en8.de>
Cc: x86@...nel.org, Andreas Herrmann <aherrmann@...e.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Chen Yu <yu.c.chen@...el.com>, Len Brown <len.brown@...el.com>,
	Radu Rendec <rrendec@...hat.com>,
	Pierre Gondois <Pierre.Gondois@....com>, Pu Wen <puwen@...on.cn>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Sudeep Holla <sudeep.holla@....com>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Will Deacon <will@...nel.org>, Zhang Rui <rui.zhang@...el.com>,
	Nikolay Borisov <nik.borisov@...e.com>,
	Huang Ying <ying.huang@...el.com>,
	Ricardo Neri <ricardo.neri@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 1/3] cacheinfo: Allocate memory during CPU hotplug if
 not done from the primary CPU

On Tue, Oct 08, 2024 at 05:51:06PM +0200, Borislav Petkov wrote:
> On Fri, Sep 13, 2024 at 01:31:53AM -0700, Ricardo Neri wrote:
> > The motivation for commit 5944ce092b97 was to prevent a BUG splat in
> > PREEMPT_RT kernels during memory allocation. This splat is not observed on
> > x86 because the memory allocation for cacheinfo happens in
> > detect_cache_attributes() from the cacheinfo CPU hotplug callback.
> > 
> > The dereference of a NULL cacheinfo is not observed today because
> > cache_leaves(cpu) is zero until after init_cache_level() is called
> > (during the CPU hotplug callback). A subsequent changeset will set
> > the number of cache leaves earlier and the NULL-pointer dereference
> > will be observed.
> 
> Lemme get this straight: this NULL ptr deref will happen after your changes

Yes. It does happen only after my changes.

> so we don't really need this patch in stable and thus no Fixes: tag at all,
> right?

IMHO, this patchset is needed in stable kernel because /sys/devices/system/cpu
cpu0/cache would be broken on Meteor Lake and processors like it.

This patch alone is not needed in stable kernels.

Thanks and BR,
Ricardo