linux-kernel - [PATCH 4/7] ARM: Use hack from powerpc to get current cpu number

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Date:   Tue,  7 Sep 2021 15:00:35 -0700
From:   Keith Packard <keithpac@...zon.com>
To:     linux-kernel@...r.kernel.org
Cc:     Abbott Liu <liuwenliang@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Christoph Lameter <cl@...ux.com>,
        Dennis Zhou <dennis@...nel.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Jens Axboe <axboe@...nel.dk>, Joe Perches <joe@...ches.com>,
        Kees Cook <keescook@...omium.org>,
        Keith Packard <keithpac@...zon.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
        Manivannan Sadhasivam <mani@...nel.org>,
        Marc Zyngier <maz@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Mike Rapoport <rppt@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nick Desaulniers <ndesaulniers@...ogle.com>,
        Nicolas Pitre <nico@...xnic.net>,
        Russell King <linux@...linux.org.uk>,
        Tejun Heo <tj@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Valentin Schneider <valentin.schneider@....com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        "Wolfram Sang (Renesas)" <wsa+renesas@...g-engineering.com>,
        YiFei Zhu <yifeifz2@...inois.edu>
Subject: [PATCH 4/7] ARM: Use hack from powerpc to get current cpu number

When we enable THREAD_INFO_IN_TASK, the cpu number will disappear from
thread_info and reappear in task_struct. As we cannot include
linux/sched.h in asm/smp.h, there's no way to use that struct type in
the raw_smp_processor_id macro. Instead, a hack from the powerpc code
is used. This pulls the TI_CPU offset out of asm-offsets.h and uses
that to find the cpu value.

Signed-off-by: Keith Packard <keithpac@...zon.com>
---
 arch/arm/Makefile          |  8 ++++++++
 arch/arm/include/asm/smp.h | 18 +++++++++++++++++-
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index 415c3514573a..6752995d2914 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -284,6 +284,14 @@ stack_protector_prepare: prepare0
 	$(eval GCC_PLUGINS_CFLAGS += $(SSP_PLUGIN_CFLAGS))
 endif
 
+ifdef CONFIG_SMP
+prepare: task_cpu_prepare
+
+PHONY += task_cpu_prepare
+task_cpu_prepare: prepare0
+	$(eval KBUILD_CFLAGS += -D_TI_CPU=$(shell awk '{if ($$2 == "TI_CPU") print $$3;}' include/generated/asm-offsets.h))
+endif
+
 all:	$(notdir $(KBUILD_IMAGE))
 
 
diff --git a/arch/arm/include/asm/smp.h b/arch/arm/include/asm/smp.h
index d43b64635d77..f77ba3753bc4 100644
--- a/arch/arm/include/asm/smp.h
+++ b/arch/arm/include/asm/smp.h
@@ -15,7 +15,23 @@
 # error "<asm/smp.h> included in non-SMP build"
 #endif
 
-#define raw_smp_processor_id() (current_thread_info()->cpu)
+/*
+ * This is particularly ugly: it appears we can't actually get the
+ * definition of task_struct here, but we need access to the CPU this
+ * task is running on, which is stored in task_struct when
+ * THREAD_INFO_IN_TASK is set.  Instead of using task_struct we're
+ * using TI_CPU which is extracted from asm-offsets.h by kbuild to get
+ * the current processor ID.
+ *
+ * This also needs to be safeguarded when building asm-offsets.s
+ * because at that time TI_CPU is not defined yet.
+ */
+#ifndef _TI_CPU
+#define raw_smp_processor_id()		(0)
+#else
+#define raw_smp_processor_id()	\
+	(*(unsigned int *)((void *)current_thread_info() + _TI_CPU))
+#endif
 
 struct seq_file;
 
-- 
2.33.0