Re: [PATCH v3 1/3] cacheinfo: Allocate memory for memory if not done from the primary CPU

[Radu Rendec]

On Fri, 2023-08-04 at 18:24 -0700, Ricardo Neri wrote:
> Commit 5944ce092b97 ("arch_topology: Build cacheinfo from primary CPU")
> adds functionality that architectures can use to optionally allocate and
> build cacheinfo early during boot. Commit 6539cffa9495 ("cacheinfo: Add
> arch specific early level initializer") lets secondary CPUs correct (and
> reallocate memory) cacheinfo data if needed.
> 
> If the early build functionality is not used and cacheinfo does not need
> correction, memory for cacheinfo is never allocated. x86 does not use the
> early build functionality. Consequently, during the cacheinfo CPU hotplug
> callback, last_level_cache_is_valid() attempts to dereference a NULL
> pointer:
> 
>      BUG: kernel NULL pointer dereference, address: 0000000000000100
>      #PF: supervisor read access in kernel mode
>      #PF: error_code(0x0000) - not present page
>      PGD 0 P4D 0
>      Oops: 0000 [#1] PREEPMT SMP NOPTI
>      CPU: 0 PID 19 Comm: cpuhp/0 Not tainted 6.4.0-rc2 #1
>      RIP: 0010: last_level_cache_is_valid+0x95/0xe0a
> 
> Allocate memory for cacheinfo during the cacheinfo CPU hotplug callback if
> not done earlier.
> 
> Cc: Andreas Herrmann <aherrmann@xxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Chen Yu <yu.c.chen@xxxxxxxxx>
> Cc: Len Brown <len.brown@xxxxxxxxx>
> Cc: Radu Rendec <rrendec@xxxxxxxxxx>
> Cc: Pierre Gondois <Pierre.Gondois@xxxxxxx>
> Cc: Pu Wen <puwen@xxxxxxxx>
> Cc: "Rafael J. Wysocki" <rafael.j.wysocki@xxxxxxxxx>
> Cc: Sudeep Holla <sudeep.holla@xxxxxxx>
> Cc: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Zhang Rui <rui.zhang@xxxxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> Acked-by: Len Brown <len.brown@xxxxxxxxx>
> Fixes: 6539cffa9495 ("cacheinfo: Add arch specific early level initializer")
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> ---
> 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 pointer is not observed today because
> cache_leaves(cpu) is zero until after init_cache_level() is called (also
> during the CPU hotplug callback). Patch2 will set it earlier and the NULL-
> pointer dereference will be observed.
> ---
> Changes since v2:
>  * Introduced this patch.
> 
> Changes since v1:
>  * N/A
> ---
>  drivers/base/cacheinfo.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/base/cacheinfo.c b/drivers/base/cacheinfo.c
> index cbae8be1fe52..461a77ece4b0 100644
> --- a/drivers/base/cacheinfo.c
> +++ b/drivers/base/cacheinfo.c
> @@ -554,7 +554,11 @@ static inline int init_level_allocate_ci(unsigned int cpu)
>          */
>         ci_cacheinfo(cpu)->early_ci_levels = false;
>  
> -       if (cache_leaves(cpu) <= early_leaves)
> +       /*
> +        * Some architectures (e.g., x86) do not use early initialization.
> +        * Allocate memory now in such case.
> +        */
> +       if (cache_leaves(cpu) <= early_leaves && per_cpu_cacheinfo(cpu))
>                 return 0;
>  
>         kfree(per_cpu_cacheinfo(cpu));

For this patch only:

Reviewed-by: Radu Rendec <rrendec@xxxxxxxxxx>

Thanks for submitting!

Best regards,
Radu