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: <20241008155106.GMZwVU6gjw45z2gn8x@fat_crate.local>
Date: Tue, 8 Oct 2024 17:51:06 +0200
From: Borislav Petkov <bp@...en8.de>
To: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
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 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 so
we don't really need this patch in stable and thus no Fixes: tag at all,
right?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette