Index: sys/sys/_mutex.h
===================================================================
--- sys/sys/_mutex.h	(revision 242016)
+++ sys/sys/_mutex.h	(working copy)
@@ -31,12 +31,30 @@
 #ifndef _SYS__MUTEX_H_
 #define	_SYS__MUTEX_H_
 
+#include <machine/param.h>
+
 /*
  * Sleep/spin mutex.
+ *
+ * In order to support 2 different types of structures with the same
+ * KPI, the second member of mutexes must be the lock cookie, called
+ * "mtx_lock".  Other type of locks must not use the same name for the
+ * lock cookie.
+ * An accessor macro is provided in order to extract the mutex ptr from
+ * the cookie address, assuming that the first member is lock_object
+ * as must always be for every lock.
  */
 struct mtx {
 	struct lock_object	lock_object;	/* Common lock properties. */
 	volatile uintptr_t	mtx_lock;	/* Owner and flags. */
 };
 
+struct mtx_unshare {
+	struct lock_object	lock_object;	/* Common lock properties. */
+	volatile uintptr_t	mtx_lock;	/* Owner and flags. */
+} __aligned(CACHE_LINE_SIZE);
+
+#define	MTX_FROM_COOKIE(ptr)						\
+	((struct mtx *)((ptr) - sizeof(struct lock_object)))
+
 #endif /* !_SYS__MUTEX_H_ */
Index: sys/sys/mutex.h
===================================================================
--- sys/sys/mutex.h	(revision 242016)
+++ sys/sys/mutex.h	(working copy)
@@ -89,7 +89,8 @@
  * [See below for descriptions]
  *
  */
-void	mtx_init(struct mtx *m, const char *name, const char *type, int opts);
+void	_mtx_init(volatile uintptr_t *c, const char *name, const char *type,
+	    int opts);
 void	mtx_destroy(struct mtx *m);
 void	mtx_sysinit(void *arg);
 int	mtx_trylock_flags_(struct mtx *m, int opts, const char *file,
@@ -123,6 +124,8 @@ void	thread_lock_flags_(struct thread *, int, cons
 
 #define	mtx_recurse	lock_object.lo_data
 
+#define	mtx_init(m, n, t, o)	_mtx_init(&(m)->mtx_lock, n, t, o)
+
 /* Very simple operations on mtx_lock. */
 
 /* Try to obtain mtx_lock once. */
Index: sys/kern/kern_mutex.c
===================================================================
--- sys/kern/kern_mutex.c	(revision 242016)
+++ sys/kern/kern_mutex.c	(working copy)
@@ -816,11 +816,14 @@ mtx_sysinit(void *arg)
  * witness.
  */
 void
-mtx_init(struct mtx *m, const char *name, const char *type, int opts)
+_mtx_init(volatile uintptr_t *c, const char *name, const char *type, int opts)
 {
+	struct mtx *m;
 	struct lock_class *class;
 	int flags;
 
+	m = MTX_FROM_COOKIE(c);
+
 	MPASS((opts & ~(MTX_SPIN | MTX_QUIET | MTX_RECURSE |
 		MTX_NOWITNESS | MTX_DUPOK | MTX_NOPROFILE)) == 0);
 	ASSERT_ATOMIC_LOAD_PTR(m->mtx_lock,