[PATCH] arm64: smp: Prevent raw_smp_processor_id() recursion

[Marek Szyprowski]

Hi Robin,


On 2016-12-01 16:55, Robin Murphy wrote:
> Under CONFIG_DEBUG_PREEMPT=y, this_cpu_ptr() ends up calling back into
> raw_smp_processor_id(), resulting in some hilariously catastrophic
> infinite recursion. In the normal case, we have:
>
>    #define this_cpu_ptr(ptr) raw_cpu_ptr(ptr)
>
> and everything is dandy. However for CONFIG_DEBUG_PREEMPT, this_cpu_ptr()
> is defined in terms of my_cpu_offset, wherein the fun begins:
>
>    #define my_cpu_offset per_cpu_offset(smp_processor_id())
>    ...
>    #define smp_processor_id() debug_smp_processor_id()
>    ...
>    notrace unsigned int debug_smp_processor_id(void)
>    {
>    	return check_preemption_disabled("smp_processor_id", "");
>    ...
>    notrace static unsigned int check_preemption_disabled(const char *what1,
>    							const char *what2)
>    {
>    	int this_cpu = raw_smp_processor_id();
>
> and bang. Use raw_cpu_ptr() directly to avoid that.
>
> Reported-by: Marek Szyprowski <m.szyprowski at samsung.com>
> Acked-by: Will Deacon <will.deacon at arm.com>
> Signed-off-by: Robin Murphy <robin.murphy at arm.com>

Works fine now. Thanks for the proper fix.

Tested-by: Marek Szyprowski <m.szyprowski at samsung.com>

> ---
>
> Since I just reproduced this locally to verify Will's suggestion, it
> seemed I might as well just write it up as a patch :)
>
>   arch/arm64/include/asm/smp.h | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/include/asm/smp.h b/arch/arm64/include/asm/smp.h
> index a62db952ffcb..d050d720a1b4 100644
> --- a/arch/arm64/include/asm/smp.h
> +++ b/arch/arm64/include/asm/smp.h
> @@ -41,8 +41,10 @@ DECLARE_PER_CPU_READ_MOSTLY(int, cpu_number);
>    * We don't use this_cpu_read(cpu_number) as that has implicit writes to
>    * preempt_count, and associated (compiler) barriers, that we'd like to avoid
>    * the expense of. If we're preemptible, the value can be stale at use anyway.
> + * And we can't use this_cpu_ptr() either, as that winds up recursing back
> + * here under CONFIG_DEBUG_PREEMPT=y.
>    */
> -#define raw_smp_processor_id() (*this_cpu_ptr(&cpu_number))
> +#define raw_smp_processor_id() (*raw_cpu_ptr(&cpu_number))
>   
>   struct seq_file;
>   

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland