x86/percpu: Fix "const_pcpu_hot" version generation failure

Version generation for "const_pcpu_hot" symbol failed because genksyms
doesn't know the __seg_gs keyword. Effectively revert commit 4604c05
("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.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20231204210320.114429-1-ubizjak@gmail.com

Author: ubizjak

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);
 }

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)

arch/x86/include/asm/processor.h

@@ -519,7 +519,7 @@ 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);
 }