Add MUTEX_FSTRANS mutex type

There are regions in the ZFS code where it is desirable to be able
to be set PF_FSTRANS while a specific mutex is held.  The ZFS code
could be updated to set/clear this flag in all the correct places,
but this is undesirable for a few reasons.

1) It would require changes to a significant amount of the ZFS
   code.  This would complicate applying patches from upstream.

2) It would be easy to accidentally miss a critical region in
   the initial patch or to have an future change introduce a
   new one.

Both of these concerns can be addressed by adding a new mutex type
which is responsible for managing PF_FSTRANS.  This lets us make a
a small change to the ZFS source where the mutex is initialized
and then be certain that all future use of that mutex will be safe.

Signed-off-by: Brian Behlendorf <[email protected]>

include/sys/mutex.h

@@ -26,19 +26,22 @@
 #define	_SPL_MUTEX_H
 
 #include <sys/types.h>
+#include <sys/kmem.h>
 #include <linux/mutex.h>
 #include <linux/compiler_compat.h>
 
 typedef enum {
 	MUTEX_DEFAULT	= 0,
 	MUTEX_SPIN	= 1,
-	MUTEX_ADAPTIVE	= 2
+	MUTEX_ADAPTIVE	= 2,
+	MUTEX_FSTRANS	= 3,
 } kmutex_type_t;
 
 typedef struct {
 	struct mutex		m_mutex;
 	spinlock_t		m_lock;	/* used for serializing mutex_exit */
 	kthread_t		*m_owner;
+	fstrans_cookie_t	*m_fstrans;
 } kmutex_t;
 
 #define	MUTEX(mp)		(&((mp)->m_mutex))
@@ -55,6 +58,25 @@ spl_mutex_clear_owner(kmutex_t *mp)
 	mp->m_owner = NULL;
 }
 
+/*
+ * MUTEX_FSTRANS helpers functions.  When mp->m_fstrans is set it indicates
+ * a fstrans mutex type.  These mutexes behave identically to default mutexes
+ * except they set PF_FSTRANS in the task for the critical region.
+ */
+static inline void
+spl_mutex_set_fstrans(kmutex_t *mp)
+{
+	if (mp->m_fstrans)
+		*(mp->m_fstrans) = spl_fstrans_mark();
+}
+
+static inline void
+spl_mutex_clear_fstrans(kmutex_t *mp)
+{
+	if (mp->m_fstrans)
+		spl_fstrans_unmark(*(mp->m_fstrans));
+}
+
 #define	mutex_owner(mp)		(ACCESS_ONCE((mp)->m_owner))
 #define	mutex_owned(mp)		(mutex_owner(mp) == current)
 #define	MUTEX_HELD(mp)		mutex_owned(mp)
@@ -70,11 +92,14 @@ spl_mutex_clear_owner(kmutex_t *mp)
 #define	mutex_init(mp, name, type, ibc)				\
 {								\
 	static struct lock_class_key __key;			\
-	ASSERT(type == MUTEX_DEFAULT);				\
 								\
 	__mutex_init(MUTEX(mp), #mp, &__key);			\
 	spin_lock_init(&(mp)->m_lock);				\
 	spl_mutex_clear_owner(mp);				\
+	(mp)->m_fstrans = NULL;					\
+								\
+	if (unlikely(type == MUTEX_FSTRANS))			\
+		VERIFY((mp)->m_fstrans = ibc);			\
 }
 
 #undef mutex_destroy
@@ -87,8 +112,10 @@ spl_mutex_clear_owner(kmutex_t *mp)
 ({								\
 	int _rc_;						\
 								\
-	if ((_rc_ = mutex_trylock(MUTEX(mp))) == 1)		\
+	if ((_rc_ = mutex_trylock(MUTEX(mp))) == 1) {		\
 		spl_mutex_set_owner(mp);			\
+		spl_mutex_set_fstrans(mp);			\
+	}							\
 								\
 	_rc_;							\
 })
@@ -98,6 +125,7 @@ spl_mutex_clear_owner(kmutex_t *mp)
 	ASSERT3P(mutex_owner(mp), !=, current);			\
 	mutex_lock(MUTEX(mp));					\
 	spl_mutex_set_owner(mp);				\
+	spl_mutex_set_fstrans(mp);				\
 }
 
 /*
@@ -122,6 +150,7 @@ spl_mutex_clear_owner(kmutex_t *mp)
 #define	mutex_exit(mp)						\
 {								\
 	spin_lock(&(mp)->m_lock);				\
+	spl_mutex_clear_fstrans(mp);				\
 	spl_mutex_clear_owner(mp);				\
 	mutex_unlock(MUTEX(mp));				\
 	spin_unlock(&(mp)->m_lock);				\

module/splat/splat-atomic.c

@@ -145,7 +145,7 @@ splat_atomic_test1(struct file *file, void *arg)
 
 	ap.ap_magic = SPLAT_ATOMIC_TEST_MAGIC;
 	ap.ap_file = file;
-	mutex_init(&ap.ap_lock, SPLAT_ATOMIC_TEST1_NAME, NULL, NULL);
+	mutex_init(&ap.ap_lock, SPLAT_ATOMIC_TEST1_NAME, MUTEX_DEFAULT, NULL);
 	init_waitqueue_head(&ap.ap_waitq);
 	ap.ap_atomic = SPLAT_ATOMIC_INIT_VALUE;
 	ap.ap_atomic_exited = 0;