Remove misguided HAVE_MUTEX_OWNER check

It is just plain unsafe to peek inside in-kernel
mutex structure and make assumptions about what kernel
does with those internal fields like owner.

Kernel is all too happy to stop doing the expected things
like tracing lock owner once you load a tainted module
like spl/zfs that is not GPL.

As such you will get instant assertion failures like this:

  VERIFY3(((*(volatile typeof((&((&zo->zo_lock)->m_mutex))->owner) *)&
      ((&((&zo->zo_lock)->m_mutex))->owner))) == 
     ((void *)0)) failed (ffff88030be28500 == (null))
  PANIC at zfs_onexit.c:104:zfs_onexit_destroy()
  Showing stack for process 3626
  CPU: 0 PID: 3626 Comm: mkfs.lustre Tainted: P OE ------------ 3.10.0-debug #1
  Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
  Call Trace:
  dump_stack+0x19/0x1b
  spl_dumpstack+0x44/0x50 [spl]
  spl_panic+0xbf/0xf0 [spl]
  zfs_onexit_destroy+0x17c/0x280 [zfs]
  zfsdev_release+0x48/0xd0 [zfs]

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Closes openzfs#632 
Closes openzfs#633

config/spl-build.m4

@@ -49,7 +49,6 @@ AC_DEFUN([SPL_AC_CONFIG_KERNEL], [
 	SPL_AC_USLEEP_RANGE
 	SPL_AC_KMEM_CACHE_ALLOCFLAGS
 	SPL_AC_WAIT_ON_BIT
-	SPL_AC_MUTEX_OWNER
 	SPL_AC_INODE_LOCK
 	SPL_AC_GROUP_INFO_GID
 	SPL_AC_KMEM_CACHE_CREATE_USERCOPY
@@ -1562,35 +1561,6 @@ AC_DEFUN([SPL_AC_WAIT_ON_BIT], [
 	])
 ])
 
-dnl #
-dnl # Check whether mutex has owner with task_struct type.
-dnl #
-dnl # Note that before Linux 3.0, mutex owner is of type thread_info.
-dnl #
-dnl # Note that in Linux 3.18, the condition for owner is changed from
-dnl # defined(CONFIG_DEBUG_MUTEXES) || defined(CONFIG_SMP) to
-dnl # defined(CONFIG_DEBUG_MUTEXES) || defined(CONFIG_MUTEX_SPIN_ON_OWNER)
-dnl #
-AC_DEFUN([SPL_AC_MUTEX_OWNER], [
-	AC_MSG_CHECKING([whether mutex has owner])
-	tmp_flags="$EXTRA_KCFLAGS"
-	EXTRA_KCFLAGS="-Werror"
-	SPL_LINUX_TRY_COMPILE([
-		#include <linux/mutex.h>
-		#include <linux/spinlock.h>
-	],[
-		DEFINE_MUTEX(m);
-		struct task_struct *t __attribute__ ((unused));
-		t = m.owner;
-	],[
-		AC_MSG_RESULT(yes)
-		AC_DEFINE(HAVE_MUTEX_OWNER, 1, [yes])
-	],[
-		AC_MSG_RESULT(no)
-	])
-	EXTRA_KCFLAGS="$tmp_flags"
-])
-
 dnl #
 dnl # 4.7 API change
 dnl # i_mutex is changed to i_rwsem. Instead of directly using

include/sys/mutex.h

@@ -40,10 +40,8 @@ typedef enum {
 typedef struct {
 	struct mutex		m_mutex;
 	spinlock_t		m_lock;	/* used for serializing mutex_exit */
-#ifndef HAVE_MUTEX_OWNER
 	/* only when kernel doesn't have owner */
 	kthread_t		*m_owner;
-#endif
 #ifdef CONFIG_LOCKDEP
 	kmutex_type_t		m_type;
 #endif /* CONFIG_LOCKDEP */
@@ -58,24 +56,16 @@ spl_mutex_set_owner(kmutex_t *mp)
 	 * kernel will handle its owner, so we don't need to do anything if it
 	 * is defined.
 	 */
-#ifndef HAVE_MUTEX_OWNER
 	mp->m_owner = current;
-#endif
 }
 
 static inline void
 spl_mutex_clear_owner(kmutex_t *mp)
 {
-#ifndef HAVE_MUTEX_OWNER
 	mp->m_owner = NULL;
-#endif
 }
 
-#ifdef HAVE_MUTEX_OWNER
 #define	mutex_owner(mp)		(ACCESS_ONCE(MUTEX(mp)->owner))
-#else
-#define	mutex_owner(mp)		(ACCESS_ONCE((mp)->m_owner))
-#endif
 #define	mutex_owned(mp)		(mutex_owner(mp) == current)
 #define	MUTEX_HELD(mp)		mutex_owned(mp)
 #define	MUTEX_NOT_HELD(mp)	(!MUTEX_HELD(mp))