ext2: Call dquot_writeback_dquots() with s_umount held

ext2_sync_fs() could be called without s_umount semaphore held when
called through ext2_write_super() from __ext2_write_inode(). This
function then calls dquot_writeback_dquots() which relies on s_umount to
be held for protection against other quota operations.

In fact __ext2_write_inode() does not need all the functionality
ext2_write_super() provides. It is enough to just write the superblock.
So use ext2_sync_super() instead.

Fixes: 9d1ccbe70e0b14545caad12dc73adb3605447df0
Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/ext2/ext2.h

@@ -796,7 +796,8 @@ void ext2_error(struct super_block *, const char *, const char *, ...);
 extern __printf(3, 4)
 void ext2_msg(struct super_block *, const char *, const char *, ...);
 extern void ext2_update_dynamic_rev (struct super_block *sb);
-extern void ext2_write_super (struct super_block *);
+extern void ext2_sync_super(struct super_block *sb, struct ext2_super_block *es,
+			    int wait);
 
 /*
  * Inodes and files operations

fs/ext2/inode.c

@@ -1615,7 +1615,7 @@ static int __ext2_write_inode(struct inode *inode, int do_sync)
 				EXT2_SET_RO_COMPAT_FEATURE(sb,
 					EXT2_FEATURE_RO_COMPAT_LARGE_FILE);
 				spin_unlock(&EXT2_SB(sb)->s_lock);
-				ext2_write_super(sb);
+				ext2_sync_super(sb, EXT2_SB(sb)->s_es, 1);
 			}
 		}
 	}

fs/ext2/super.c

@@ -36,8 +36,7 @@
 #include "xattr.h"
 #include "acl.h"
 
-static void ext2_sync_super(struct super_block *sb,
-			    struct ext2_super_block *es, int wait);
+static void ext2_write_super(struct super_block *sb);
 static int ext2_remount (struct super_block * sb, int * flags, char * data);
 static int ext2_statfs (struct dentry * dentry, struct kstatfs * buf);
 static int ext2_sync_fs(struct super_block *sb, int wait);
@@ -1194,8 +1193,8 @@ static void ext2_clear_super_error(struct super_block *sb)
 	}
 }
 
-static void ext2_sync_super(struct super_block *sb, struct ext2_super_block *es,
-			    int wait)
+void ext2_sync_super(struct super_block *sb, struct ext2_super_block *es,
+		     int wait)
 {
 	ext2_clear_super_error(sb);
 	spin_lock(&EXT2_SB(sb)->s_lock);
@@ -1270,7 +1269,7 @@ static int ext2_unfreeze(struct super_block *sb)
 	return 0;
 }
 
-void ext2_write_super(struct super_block *sb)
+static void ext2_write_super(struct super_block *sb)
 {
 	if (!(sb->s_flags & MS_RDONLY))
 		ext2_sync_fs(sb, 1);