linux-kernel - Re: [PATCH -next] arm64: Export __cpu_logical

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Date:   Fri, 24 Jul 2020 10:13:52 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Kefeng Wang <wangkefeng.wang@...wei.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mikko Perttunen <mperttunen@...dia.com>,
        Sumit Gupta <sumitg@...dia.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Hulk Robot <hulkci@...wei.com>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH -next] arm64: Export __cpu_logical_map

On Fri, Jul 24, 2020 at 01:46:18PM +0530, Anshuman Khandual wrote:
> 
> On 07/24/2020 08:38 AM, Kefeng Wang wrote:
> > +maillist
> 
> This does not seem to be a correct method of posting any patch.
> 
> > 
> > On 2020/7/24 11:04, Kefeng Wang wrote:
> >> ERROR: modpost: "__cpu_logical_map" [drivers/cpufreq/tegra194-cpufreq.ko] undefined!
> 
> 
> >>
> >> ARM64 tegra194-cpufreq driver use cpu_logical_map, export
> >> __cpu_logical_map to fix build issue.
> 
> Commit 887d5fc82cb4 ("cpufreq: Add Tegra194 cpufreq driver") which adds
> this particular driver is present just on linux-next. But as expected,
> the driver does not use __cpu_logical_map directly but instead accesses
> it via cpu_logical_map() wrapper. Wondering, how did you even trigger
> the modpost error ?

The wrapper in <asm/smp.h> is:

| /*
|  * Logical CPU mapping.
|  */
| extern u64 __cpu_logical_map[NR_CPUS];
| #define cpu_logical_map(cpu)    __cpu_logical_map[cpu]

... and use of that would blow up as described.

> >>
> >> Reported-by: Hulk Robot <hulkci@...wei.com>
> >> Signed-off-by: Kefeng Wang <wangkefeng.wang@...wei.com>
> >> ---
> >>   arch/arm64/kernel/setup.c | 1 +
> >>   1 file changed, 1 insertion(+)
> >>
> >> diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
> >> index c793276ec7ad9..3aea05fbb9998 100644
> >> --- a/arch/arm64/kernel/setup.c
> >> +++ b/arch/arm64/kernel/setup.c
> >> @@ -275,6 +275,7 @@ static int __init reserve_memblock_reserved_regions(void)
> >>   arch_initcall(reserve_memblock_reserved_regions);
> >>     u64 __cpu_logical_map[NR_CPUS] = { [0 ... NR_CPUS-1] = INVALID_HWID };
> >> +EXPORT_SYMBOL(__cpu_logical_map);

If modules are using cpu_logical_map(), this looks sane ot me, but I
wonder if we should instead turn cpu_logical_map() into a C wrapper in
smp.c, or at least mark __cpu_logical_map as __ro_after_init lest
someone have the bright idea to fiddle with it.

Mark.