f2fs: introduce sb_lock to make encrypt pwsalt update exclusive

f2fs_super_block.encrypt_pw_salt can be udpated and persisted
concurrently, result in getting different pwsalt in separated
threads, so let's introduce sb_lock to exclude concurrent
accessers.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/f2fs.h

@@ -1047,6 +1047,7 @@ struct f2fs_sb_info {
 	struct super_block *sb;			/* pointer to VFS super block */
 	struct proc_dir_entry *s_proc;		/* proc entry */
 	struct f2fs_super_block *raw_super;	/* raw super block pointer */
+	struct mutex sb_lock;			/* lock for raw super block */
 	int valid_super_block;			/* valid super block no */
 	unsigned long s_flag;				/* flags for sbi */
 

fs/f2fs/file.c

@@ -1962,13 +1962,15 @@ static int f2fs_ioc_get_encryption_pwsalt(struct file *filp, unsigned long arg)
 	if (!f2fs_sb_has_encrypt(inode->i_sb))
 		return -EOPNOTSUPP;
 
-	if (uuid_is_nonzero(sbi->raw_super->encrypt_pw_salt))
-		goto got_it;
-
 	err = mnt_want_write_file(filp);
 	if (err)
 		return err;
 
+	mutex_lock(&sbi->sb_lock);
+
+	if (uuid_is_nonzero(sbi->raw_super->encrypt_pw_salt))
+		goto got_it;
+
 	/* update superblock with uuid */
 	generate_random_uuid(sbi->raw_super->encrypt_pw_salt);
 
@@ -1976,15 +1978,16 @@ static int f2fs_ioc_get_encryption_pwsalt(struct file *filp, unsigned long arg)
 	if (err) {
 		/* undo new data */
 		memset(sbi->raw_super->encrypt_pw_salt, 0, 16);
-		mnt_drop_write_file(filp);
-		return err;
+		goto out_err;
 	}
-	mnt_drop_write_file(filp);
 got_it:
 	if (copy_to_user((__u8 __user *)arg, sbi->raw_super->encrypt_pw_salt,
 									16))
-		return -EFAULT;
-	return 0;
+		err = -EFAULT;
+out_err:
+	mutex_unlock(&sbi->sb_lock);
+	mnt_drop_write_file(filp);
+	return err;
 }
 
 static int f2fs_ioc_gc(struct file *filp, unsigned long arg)

fs/f2fs/super.c

@@ -2221,6 +2221,8 @@ static void init_sb_info(struct f2fs_sb_info *sbi)
 
 	sbi->dirty_device = 0;
 	spin_lock_init(&sbi->dev_lock);
+
+	mutex_init(&sbi->sb_lock);
 }
 
 static int init_percpu_info(struct f2fs_sb_info *sbi)