linux-kernel - FAILED: Patch "LoongArch: Change __my_cpu_offset definition to avoid mis-optimization" failed to apply to 4.19-stable tree

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Date: Wed, 27 Mar 2024 08:26:05 -0400
From: Sasha Levin <sashal@...nel.org>
To: stable@...r.kernel.org,
	chenhuacai@...nel.org
Cc: 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>,
	Huacai Chen <chenhuacai@...ngson.cn>,
	linux-mm@...ck.org,
	loongarch@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: FAILED: Patch "LoongArch: Change __my_cpu_offset definition to avoid mis-optimization" failed to apply to 4.19-stable tree

The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@...r.kernel.org>.

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

>From c87e12e0e8c1241410e758e181ca6bf23efa5b5b Mon Sep 17 00:00:00 2001
From: Huacai Chen <chenhuacai@...ngson.cn>
Date: Tue, 19 Mar 2024 15:50:34 +0800
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 | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/loongarch/include/asm/percpu.h b/arch/loongarch/include/asm/percpu.h
index 9b36ac003f890..8f290e5546cf7 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