locking: Detect includes rwlock.h outside of spinlock.h

From: Michael S. Tsirkin <mst@redhat.com>

The check for __LINUX_SPINLOCK_H within rwlock.h (and other files)
detects the direct include of the header file if it is at the very
beginning of the include section.
If it is listed later then chances are high that spinlock.h was already
included (including rwlock.h) and the additional listing of rwlock.h
will not cause any failure.

On PREEMPT_RT this additional rwlock.h will lead to compile failures
since it uses a different rwlock implementation.

Add __LINUX_INSIDE_SPINLOCK_H to spinlock.h and check for this instead
of __LINUX_SPINLOCK_H to detect wrong includes. This will help detect
direct includes of rwlock.h with without PREEMPT_RT enabled.

[ bigeasy: add remaining __LINUX_SPINLOCK_H user and rewrite
  commit description. ]
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/YweemHxJx7O8rjBx@linutronix.de

include/linux/rwlock.h

 #ifndef __LINUX_RWLOCK_H
 #define __LINUX_RWLOCK_H
 
-#ifndef __LINUX_SPINLOCK_H
+#ifndef __LINUX_INSIDE_SPINLOCK_H
 # error "please don't include this file directly"
 #endif
 

include/linux/spinlock.h

 /* SPDX-License-Identifier: GPL-2.0 */
 #ifndef __LINUX_SPINLOCK_H
 #define __LINUX_SPINLOCK_H
+#define __LINUX_INSIDE_SPINLOCK_H
 
 /*
  * include/linux/spinlock.h - generic spinlock/rwlock declarations
@@ -492,4 +493,5 @@ int __alloc_bucket_spinlocks(spinlock_t **locks, unsigned int *lock_mask,
 
 void free_bucket_spinlocks(spinlock_t *locks);
 
+#undef __LINUX_INSIDE_SPINLOCK_H
 #endif /* __LINUX_SPINLOCK_H */

include/linux/spinlock_api_smp.h

 #ifndef __LINUX_SPINLOCK_API_SMP_H
 #define __LINUX_SPINLOCK_API_SMP_H
 
-#ifndef __LINUX_SPINLOCK_H
+#ifndef __LINUX_INSIDE_SPINLOCK_H
 # error "please don't include this file directly"
 #endif
 

include/linux/spinlock_api_up.h

 #ifndef __LINUX_SPINLOCK_API_UP_H
 #define __LINUX_SPINLOCK_API_UP_H
 
-#ifndef __LINUX_SPINLOCK_H
+#ifndef __LINUX_INSIDE_SPINLOCK_H
 # error "please don't include this file directly"
 #endif
 

include/linux/spinlock_rt.h

@@ -2,7 +2,7 @@
 #ifndef __LINUX_SPINLOCK_RT_H
 #define __LINUX_SPINLOCK_RT_H
 
-#ifndef __LINUX_SPINLOCK_H
+#ifndef __LINUX_INSIDE_SPINLOCK_H
 #error Do not include directly. Use spinlock.h
 #endif
 

include/linux/spinlock_up.h

 #ifndef __LINUX_SPINLOCK_UP_H
 #define __LINUX_SPINLOCK_UP_H
 
-#ifndef __LINUX_SPINLOCK_H
+#ifndef __LINUX_INSIDE_SPINLOCK_H
 # error "please don't include this file directly"
 #endif