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

[Ricardo Neri]

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