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>

2 files changed, 12 insertions, 9 deletions

diff --git a/fs/nilfs2/ioctl.c b/fs/nilfs2/ioctl.c
index 108d281ebca5..be387c6b2d46 100644
--- a/fs/nilfs2/ioctl.c
+++ b/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;
 }
diff --git a/fs/nilfs2/mdt.c b/fs/nilfs2/mdt.c
index 47dd815433fd..e1c6777931b7 100644
--- a/fs/nilfs2/mdt.c
+++ b/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;
 }