nilfs2: fix circular locking dependency of writer mutex

This fixes the following circular locking dependency problem:

 =======================================================
 [ INFO: possible circular locking dependency detected ]
 2.6.30-rc3 #5
 -------------------------------------------------------
 segctord/3895 is trying to acquire lock:
  (&nilfs->ns_writer_mutex){+.+...}, at: [<d0d02172>]
   nilfs_mdt_get_block+0x89/0x20f [nilfs2]

 but task is already holding lock:
  (&bmap->b_sem){++++..}, at: [<d0d02d99>]
   nilfs_bmap_propagate+0x14/0x2e [nilfs2]

 which lock already depends on the new lock.

The bugfix is done by replacing call sites of nilfs_get_writer() which
are never called from read-only context with direct dereferencing of
pointer to a writable FS-instance.
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

fs/nilfs2/ioctl.c

@@ -516,14 +516,16 @@ static ssize_t
 nilfs_ioctl_do_free_segments(struct the_nilfs *nilfs, __u64 *posp, int flags,
 			     void *buf, size_t size, size_t nmembs)
 {
-	struct nilfs_sb_info *sbi = nilfs_get_writer(nilfs);
+	struct nilfs_sb_info *sbi = nilfs->ns_writer;
 	int ret;
 
-	if (unlikely(!sbi))
+	if (unlikely(!sbi)) {
+		/* never happens because called for a writable mount */
+		WARN_ON(1);
 		return -EROFS;
+	}
 	ret = nilfs_segctor_add_segments_to_be_freed(
 		NILFS_SC(sbi), buf, nmembs);
-	nilfs_put_writer(nilfs);
 
 	return (ret < 0) ? ret : nmembs;
 }

fs/nilfs2/mdt.c

@@ -77,19 +77,22 @@ static int nilfs_mdt_create_block(struct inode *inode, unsigned long block,
 						     void *))
 {
 	struct the_nilfs *nilfs = NILFS_MDT(inode)->mi_nilfs;
-	struct nilfs_sb_info *writer = NULL;
 	struct super_block *sb = inode->i_sb;
 	struct nilfs_transaction_info ti;
 	struct buffer_head *bh;
 	int err;
 
 	if (!sb) {
-		writer = nilfs_get_writer(nilfs);
-		if (!writer) {
+		/*
+		 * Make sure this function is not called from any
+		 * read-only context.
+		 */
+		if (!nilfs->ns_writer) {
+			WARN_ON(1);
 			err = -EROFS;
 			goto out;
 		}
-		sb = writer->s_super;
+		sb = nilfs->ns_writer->s_super;
 	}
 
 	nilfs_transaction_begin(sb, &ti, 0);
@@ -127,8 +130,6 @@ static int nilfs_mdt_create_block(struct inode *inode, unsigned long block,
 		err = nilfs_transaction_commit(sb);
 	else
 		nilfs_transaction_abort(sb);
-	if (writer)
-		nilfs_put_writer(nilfs);
  out:
 	return err;
 }