[PATCH -tip v3 1/3] x86/percpu: Fix "const_pcpu_hot" version generation failure

[Uros Bizjak]

Version generation for "const_pcpu_hot" symbol failed because genksyms
doesn't know the __seg_gs keyword. Revert commit 4604c052b84d
"x86/percpu: Declare const_pcpu_hot as extern const variable" and
use this_cpu_read_const instead to avoid "sparse: dereference of
noderef expression" warning when reading const_pcpu_hot.

Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
---
v2: Use BUILD_BUG() instead of BUG().
---
 arch/x86/include/asm/current.h   | 5 +++--
 arch/x86/include/asm/percpu.h    | 7 +++++++
 arch/x86/include/asm/processor.h | 2 +-
 3 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/current.h b/arch/x86/include/asm/current.h
index 9fbd7cb2dc86..c8c5674d69f6 100644
--- a/arch/x86/include/asm/current.h
+++ b/arch/x86/include/asm/current.h
@@ -37,12 +37,13 @@ static_assert(sizeof(struct pcpu_hot) == 64);
 DECLARE_PER_CPU_ALIGNED(struct pcpu_hot, pcpu_hot);
 
 /* const-qualified alias to pcpu_hot, aliased by linker. */
-extern const struct pcpu_hot __percpu_seg_override const_pcpu_hot;
+DECLARE_PER_CPU_ALIGNED(const struct pcpu_hot __percpu_seg_override,
+			const_pcpu_hot);
 
 static __always_inline struct task_struct *get_current(void)
 {
 	if (IS_ENABLED(CONFIG_USE_X86_SEG_SUPPORT))
-		return const_pcpu_hot.current_task;
+		return this_cpu_read_const(const_pcpu_hot.current_task);
 
 	return this_cpu_read_stable(pcpu_hot.current_task);
 }
diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index 0f12b2004b94..3859abad19ec 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -28,6 +28,7 @@
 
 #else /* ...!ASSEMBLY */
 
+#include <linux/build_bug.h>
 #include <linux/kernel.h>
 #include <linux/stringify.h>
 
@@ -462,6 +463,7 @@ do {									\
 #define this_cpu_write_8(pcp, val)	__raw_cpu_write(volatile, pcp, val)
 #endif
 
+#define this_cpu_read_const(pcp)	__raw_cpu_read(, pcp)
 #else /* CONFIG_USE_X86_SEG_SUPPORT */
 
 #define raw_cpu_read_1(pcp)		percpu_from_op(1, , "mov", pcp)
@@ -486,6 +488,11 @@ do {									\
 #define this_cpu_write_8(pcp, val)	percpu_to_op(8, volatile, "mov", (pcp), val)
 #endif
 
+/*
+ * The generic per-cpu infrastrucutre is not suitable for
+ * reading const-qualified variables.
+ */
+#define this_cpu_read_const(pcp)	({ BUILD_BUG(); (typeof(pcp))0; })
 #endif /* CONFIG_USE_X86_SEG_SUPPORT */
 
 #define raw_cpu_add_1(pcp, val)		percpu_add_op(1, , (pcp), val)
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 061aa86b4662..1188e8bf76a2 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -534,7 +534,7 @@ static __always_inline unsigned long current_top_of_stack(void)
 	 *  entry trampoline.
 	 */
 	if (IS_ENABLED(CONFIG_USE_X86_SEG_SUPPORT))
-		return const_pcpu_hot.top_of_stack;
+		return this_cpu_read_const(const_pcpu_hot.top_of_stack);
 
 	return this_cpu_read_stable(pcpu_hot.top_of_stack);
 }
-- 
2.42.0