x86/fsgsbase/64: Clean up various details

So:

 - use 'extern' consistently for APIs

 - fix weird header guard

 - clarify code comments

 - reorder APIs by type

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Chang S. Bae <chang.seok.bae@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Markus T Metzger <markus.t.metzger@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Shankar <ravi.v.shankar@intel.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/1537312139-5580-2-git-send-email-chang.seok.bae@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/entry/vdso/vgetcpu.c

@@ -14,6 +14,7 @@ notrace long
 __vdso_getcpu(unsigned *cpu, unsigned *node, struct getcpu_cache *unused)
 {
 	vdso_read_cpunode(cpu, node);
+
 	return 0;
 }
 

arch/x86/include/asm/fsgsbase.h

@@ -1,6 +1,6 @@
 /* SPDX-License-Identifier: GPL-2.0 */
 #ifndef _ASM_FSGSBASE_H
-#define _ASM_FSGSBASE_H 1
+#define _ASM_FSGSBASE_H
 
 #ifndef __ASSEMBLY__
 
@@ -9,14 +9,15 @@
 #include <asm/msr-index.h>
 
 /*
- * Read/write a task's fsbase or gsbase. This returns the value that
+ * Read/write a task's FSBASE or GSBASE. This returns the value that
  * the FS/GS base would have (if the task were to be resumed). These
- * work on current or on a different non-running task.
+ * work on the current task or on a non-running (typically stopped
+ * ptrace child) task.
  */
-unsigned long x86_fsbase_read_task(struct task_struct *task);
-unsigned long x86_gsbase_read_task(struct task_struct *task);
-int x86_fsbase_write_task(struct task_struct *task, unsigned long fsbase);
-int x86_gsbase_write_task(struct task_struct *task, unsigned long gsbase);
+extern unsigned long x86_fsbase_read_task(struct task_struct *task);
+extern unsigned long x86_gsbase_read_task(struct task_struct *task);
+extern int x86_fsbase_write_task(struct task_struct *task, unsigned long fsbase);
+extern int x86_gsbase_write_task(struct task_struct *task, unsigned long gsbase);
 
 /* Helper functions for reading/writing FS/GS base */
 
@@ -25,20 +26,21 @@ static inline unsigned long x86_fsbase_read_cpu(void)
 	unsigned long fsbase;
 
 	rdmsrl(MSR_FS_BASE, fsbase);
+
 	return fsbase;
 }
 
-void x86_fsbase_write_cpu(unsigned long fsbase);
-
 static inline unsigned long x86_gsbase_read_cpu_inactive(void)
 {
 	unsigned long gsbase;
 
 	rdmsrl(MSR_KERNEL_GS_BASE, gsbase);
+
 	return gsbase;
 }
 
-void x86_gsbase_write_cpu_inactive(unsigned long gsbase);
+extern void x86_fsbase_write_cpu(unsigned long fsbase);
+extern void x86_gsbase_write_cpu_inactive(unsigned long gsbase);
 
 #endif /* CONFIG_X86_64 */