linux-kernel - [PATCH] LoongArch: Change __my_cpu_offset definition to avoid mis-optimization

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Message-ID: <20240315024526.394772-1-chenhuacai@loongson.cn>
Date: Fri, 15 Mar 2024 10:45:26 +0800
From: Huacai Chen <chenhuacai@...ngson.cn>
To: Arnd Bergmann <arnd@...db.de>,
	Huacai Chen <chenhuacai@...nel.org>
Cc: loongarch@...ts.linux.dev,
	linux-arch@...r.kernel.org,
	Xuefeng Li <lixuefeng@...ngson.cn>,
	Guo Ren <guoren@...nel.org>,
	Xuerui Wang <kernel@...0n.name>,
	Jiaxun Yang <jiaxun.yang@...goat.com>,
	linux-kernel@...r.kernel.org,
	loongson-kernel@...ts.loongnix.cn,
	Huacai Chen <chenhuacai@...ngson.cn>,
	stable@...r.kernel.org,
	Xiaotian Wu <wuxiaotian@...ngson.cn>,
	Miao Wang <shankerwangmiao@...il.com>,
	Xing Li <lixing@...ngson.cn>,
	Hongchen Zhang <zhanghongchen@...ngson.cn>,
	Rui Wang <wangrui@...ngson.cn>
Subject: [PATCH] LoongArch: Change __my_cpu_offset definition to avoid mis-optimization

>From GCC commit 3f13154553f8546a ("df-scan: remove ad-hoc handling of
global regs in asms"), global registers will no longer be forced to add
to the def-use chain. Then current_thread_info(), current_stack_pointer
and __my_cpu_offset may be lifted out of the loop because they are no
longer treated as "volatile variables".

This optimization is still correct for the current_thread_info() and
current_stack_pointer usages because they are associated to a thread.
However it is wrong for __my_cpu_offset because it is associated to a
CPU rather than a thread: if the thread migrates to a different CPU in
the loop, __my_cpu_offset should be changed.

Change __my_cpu_offset definition to treat it as a "volatile variable",
in order to avoid such a mis-optimization.

Cc: stable@...r.kernel.org
Reported-by: Xiaotian Wu <wuxiaotian@...ngson.cn>
Reported-by: Miao Wang <shankerwangmiao@...il.com>
Signed-off-by: Xing Li <lixing@...ngson.cn>
Signed-off-by: Hongchen Zhang <zhanghongchen@...ngson.cn>
Signed-off-by: Rui Wang <wangrui@...ngson.cn>
Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>
---
 arch/loongarch/include/asm/percpu.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/loongarch/include/asm/percpu.h b/arch/loongarch/include/asm/percpu.h
index 9b36ac003f89..03b98491d301 100644
--- a/arch/loongarch/include/asm/percpu.h
+++ b/arch/loongarch/include/asm/percpu.h
@@ -29,7 +29,12 @@ static inline void set_my_cpu_offset(unsigned long off)
 	__my_cpu_offset = off;
 	csr_write64(off, PERCPU_BASE_KS);
 }
-#define __my_cpu_offset __my_cpu_offset
+
+#define __my_cpu_offset					\
+({							\
+	__asm__ __volatile__("":"+r"(__my_cpu_offset));	\
+	__my_cpu_offset;				\
+})
 
 #define PERCPU_OP(op, asm_op, c_op)					\
 static __always_inline unsigned long __percpu_##op(void *ptr,		\
-- 
2.43.0