get rid of S_BIAS

use atomic_inc_not_zero(&sb->s_active) instead of playing games with
checking ->s_count > S_BIAS
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/notify/inotify/inotify.c

@@ -110,14 +110,10 @@ EXPORT_SYMBOL_GPL(get_inotify_watch);
 int pin_inotify_watch(struct inotify_watch *watch)
 {
 	struct super_block *sb = watch->inode->i_sb;
-	spin_lock(&sb_lock);
-	if (sb->s_count >= S_BIAS) {
-		atomic_inc(&sb->s_active);
-		spin_unlock(&sb_lock);
+	if (atomic_inc_not_zero(&sb->s_active)) {
 		atomic_inc(&watch->count);
 		return 1;
 	}
-	spin_unlock(&sb_lock);
 	return 0;
 }
 
@@ -518,16 +514,16 @@ EXPORT_SYMBOL_GPL(inotify_init_watch);
  * ->s_umount, which will almost certainly wait until the superblock is shut
  * down and the watch in question is pining for fjords.  That's fine, but
  * there is a problem - we might have hit the window between ->s_active
- * getting to 0 / ->s_count - below S_BIAS (i.e. the moment when superblock
- * is past the point of no return and is heading for shutdown) and the
- * moment when deactivate_super() acquires ->s_umount.  We could just do
- * drop_super() yield() and retry, but that's rather antisocial and this
- * stuff is luser-triggerable.  OTOH, having grabbed ->s_umount and having
- * found that we'd got there first (i.e. that ->s_root is non-NULL) we know
- * that we won't race with inotify_umount_inodes().  So we could grab a
- * reference to watch and do the rest as above, just with drop_super() instead
- * of deactivate_super(), right?  Wrong.  We had to drop ih->mutex before we
- * could grab ->s_umount.  So the watch could've been gone already.
+ * getting to 0 (i.e. the moment when superblock is past the point of no return
+ * and is heading for shutdown) and the moment when deactivate_super() acquires
+ * ->s_umount.  We could just do drop_super() yield() and retry, but that's
+ * rather antisocial and this stuff is luser-triggerable.  OTOH, having grabbed
+ * ->s_umount and having found that we'd got there first (i.e. that ->s_root is
+ * non-NULL) we know that we won't race with inotify_umount_inodes().  So we
+ * could grab a reference to watch and do the rest as above, just with
+ * drop_super() instead of deactivate_super(), right?  Wrong.  We had to drop
+ * ih->mutex before we could grab ->s_umount.  So the watch could've been gone
+ * already.
  *
  * That still can be dealt with - we need to save watch->wd, do idr_find()
  * and compare its result with our pointer.  If they match, we either have
@@ -565,14 +561,12 @@ static int pin_to_kill(struct inotify_handle *ih, struct inotify_watch *watch)
 	struct super_block *sb = watch->inode->i_sb;
 	s32 wd = watch->wd;
 
-	spin_lock(&sb_lock);
-	if (sb->s_count >= S_BIAS) {
-		atomic_inc(&sb->s_active);
-		spin_unlock(&sb_lock);
+	if (atomic_inc_not_zero(&sb->s_active)) {
 		get_inotify_watch(watch);
 		mutex_unlock(&ih->mutex);
 		return 1;	/* the best outcome */
 	}
+	spin_lock(&sb_lock);
 	sb->s_count++;
 	spin_unlock(&sb_lock);
 	mutex_unlock(&ih->mutex); /* can't grab ->s_umount under it */

fs/super.c

@@ -93,7 +93,7 @@ static struct super_block *alloc_super(struct file_system_type *type)
 		 * subclass.
 		 */
 		down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING);
-		s->s_count = S_BIAS;
+		s->s_count = 1;
 		atomic_set(&s->s_active, 1);
 		mutex_init(&s->s_vfs_rename_mutex);
 		mutex_init(&s->s_dquot.dqio_mutex);
@@ -189,9 +189,7 @@ void put_super(struct super_block *sb)
 void deactivate_super(struct super_block *s)
 {
 	struct file_system_type *fs = s->s_type;
-	if (atomic_dec_and_lock(&s->s_active, &sb_lock)) {
-		s->s_count -= S_BIAS-1;
-		spin_unlock(&sb_lock);
+	if (atomic_dec_and_test(&s->s_active)) {
 		vfs_dq_off(s, 0);
 		down_write(&s->s_umount);
 		fs->kill_sb(s);
@@ -216,9 +214,7 @@ EXPORT_SYMBOL(deactivate_super);
 void deactivate_locked_super(struct super_block *s)
 {
 	struct file_system_type *fs = s->s_type;
-	if (atomic_dec_and_lock(&s->s_active, &sb_lock)) {
-		s->s_count -= S_BIAS-1;
-		spin_unlock(&sb_lock);
+	if (atomic_dec_and_test(&s->s_active)) {
 		vfs_dq_off(s, 0);
 		fs->kill_sb(s);
 		put_filesystem(fs);
@@ -243,21 +239,19 @@ EXPORT_SYMBOL(deactivate_locked_super);
  */
 static int grab_super(struct super_block *s) __releases(sb_lock)
 {
+	if (atomic_inc_not_zero(&s->s_active)) {
+		spin_unlock(&sb_lock);
+		down_write(&s->s_umount);
+		return 1;
+	}
+	/* it's going away */
 	s->s_count++;
 	spin_unlock(&sb_lock);
+	/* usually that'll be enough for it to die... */
 	down_write(&s->s_umount);
-	if (s->s_root) {
-		spin_lock(&sb_lock);
-		if (s->s_count > S_BIAS) {
-			atomic_inc(&s->s_active);
-			s->s_count--;
-			spin_unlock(&sb_lock);
-			return 1;
-		}
-		spin_unlock(&sb_lock);
-	}
 	up_write(&s->s_umount);
 	put_super(s);
+	/* ... but in case it wasn't, let's at least yield() */
 	yield();
 	return 0;
 }

include/linux/fs.h

@@ -1314,7 +1314,6 @@ extern int send_sigurg(struct fown_struct *fown);
 extern struct list_head super_blocks;
 extern spinlock_t sb_lock;
 
-#define S_BIAS (1<<30)
 struct super_block {
 	struct list_head	s_list;		/* Keep this first */
 	dev_t			s_dev;		/* search index; _not_ kdev_t */