sysfs: Add lockdep annotations for the sysfs active reference

Holding locks over device_del -> kobject_del -> sysfs_deactivate can
cause deadlocks if those same locks are grabbed in sysfs show or store
methods.

The I model s_active count + completion as a sleeping read/write lock.
I describe to lockdep sysfs_get_active as a read_trylock,
sysfs_put_active as a read_unlock, and sysfs_deactivate as a
write_lock and write_unlock pair. This seems to capture the essence
for purposes of finding deadlocks, and in my testing gives finds real
issues and ignores non-issues.

This brings us back to holding locks over kobject_del is a problem
that ideally we should find a way of addressing, but at least lockdep
can tell us about the problems instead of requiring developers to debug
rare strange system deadlocks, that happen when sysfs files are removed
while being written to.

Signed-off-by: Eric W. Biederman <[e-mail]>
Acked-by: Tejun Heo <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/sysfs/dir.c

···
 			return NULL;
 
 		t = atomic_cmpxchg(&sd->s_active, v, v + 1);
-		if (likely(t == v))
+		if (likely(t == v)) {
+			rwsem_acquire_read(&sd->dep_map, 0, 1, _RET_IP_);
 			return sd;
+		}
 		if (t < 0)
 			return NULL;
 
···
 	if (unlikely(!sd))
 		return;
 
+	rwsem_release(&sd->dep_map, 1, _RET_IP_);
 	v = atomic_dec_return(&sd->s_active);
 	if (likely(v != SD_DEACTIVATED_BIAS))
 		return;
···
 	BUG_ON(sd->s_sibling || !(sd->s_flags & SYSFS_FLAG_REMOVED));
 	sd->s_sibling = (void *)&wait;
 
+	rwsem_acquire(&sd->dep_map, 0, 0, _RET_IP_);
 	/* atomic_add_return() is a mb(), put_active() will always see
 	 * the updated sd->s_sibling.
 	 */
 	v = atomic_add_return(SD_DEACTIVATED_BIAS, &sd->s_active);
 
-	if (v != SD_DEACTIVATED_BIAS)
+	if (v != SD_DEACTIVATED_BIAS) {
+		lock_contended(&sd->dep_map, _RET_IP_);
 		wait_for_completion(&wait);
+	}
 
 	sd->s_sibling = NULL;
+
+	lock_acquired(&sd->dep_map, _RET_IP_);
+	rwsem_release(&sd->dep_map, 1, _RET_IP_);
 }
 
 static int sysfs_alloc_ino(ino_t *pino)
···
 
 	atomic_set(&sd->s_count, 1);
 	atomic_set(&sd->s_active, 0);
+	sysfs_dirent_init_lockdep(sd);
 
 	sd->s_name = name;
 	sd->s_mode = mode;

fs/sysfs/sysfs.h

···
  * This file is released under the GPLv2.
  */
 
+#include <linux/lockdep.h>
 #include <linux/fs.h>
 
 struct sysfs_open_dirent;
···
 struct sysfs_dirent {
 	atomic_t		s_count;
 	atomic_t		s_active;
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+	struct lockdep_map	dep_map;
+#endif
 	struct sysfs_dirent	*s_parent;
 	struct sysfs_dirent	*s_sibling;
 	const char		*s_name;
···
 {
 	return sd->s_flags & SYSFS_TYPE_MASK;
 }
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+#define sysfs_dirent_init_lockdep(sd)				\
+do {								\
+	static struct lock_class_key __key;			\
+								\
+	lockdep_init_map(&sd->dep_map, "s_active", &__key, 0);	\
+} while(0)
+#else
+#define sysfs_dirent_init_lockdep(sd) do {} while(0)
+#endif
 
 /*
  * Context structure to be used while adding/removing nodes.