nilfs2: use semaphore to protect pointer to a writable FS-instance

will get rid of nilfs_get_writer() and nilfs_put_writer() pair used to
retain a writable FS-instance for a period.

The pair functions were making up some kind of recursive lock with a
mutex, but they became overkill since the commit
201913ed746c7724a40d33ee5a0b6a1fd2ef3193.  Furthermore, they caused
the following lockdep warning because the mutex can be released by a
task which didn't lock it:

 =====================================
 [ BUG: bad unlock balance detected! ]
 -------------------------------------
 kswapd0/422 is trying to release lock (&nilfs->ns_writer_mutex) at:
 [<c1359ff5>] mutex_unlock+0x8/0xa
 but there are no more locks to release!

 other info that might help us debug this:
 no locks held by kswapd0/422.

 stack backtrace:
 Pid: 422, comm: kswapd0 Not tainted 2.6.31-rc4-nilfs #51
 Call Trace:
  [<c1358f97>] ? printk+0xf/0x18
  [<c104fea7>] print_unlock_inbalance_bug+0xcc/0xd7
  [<c11578de>] ? prop_put_global+0x3/0x35
  [<c1050195>] lock_release+0xed/0x1dc
  [<c1359ff5>] ? mutex_unlock+0x8/0xa
  [<c1359f83>] __mutex_unlock_slowpath+0xaf/0x119
  [<c1359ff5>] mutex_unlock+0x8/0xa
  [<d1284add>] nilfs_mdt_write_page+0xd8/0xe1 [nilfs2]
  [<c1092653>] shrink_page_list+0x379/0x68d
  [<c109171b>] ? isolate_pages_global+0xb4/0x18c
  [<c1092bd2>] shrink_list+0x26b/0x54b
  [<c10930be>] shrink_zone+0x20c/0x2a2
  [<c10936b7>] kswapd+0x407/0x591
  [<c1091667>] ? isolate_pages_global+0x0/0x18c
  [<c1040603>] ? autoremove_wake_function+0x0/0x33
  [<c10932b0>] ? kswapd+0x0/0x591
  [<c104033b>] kthread+0x69/0x6e
  [<c10402d2>] ? kthread+0x0/0x6e
  [<c1003e33>] kernel_thread_helper+0x7/0x1a

This patch uses a reader/writer semaphore instead of the own lock and
kills this warning.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

3 files changed, 12 insertions, 26 deletions

diff --git a/fs/nilfs2/mdt.c b/fs/nilfs2/mdt.c
index 2dfd47714ae5..a5cd80162dc5 100644
--- a/fs/nilfs2/mdt.c
+++ b/fs/nilfs2/mdt.c
@@ -402,6 +402,7 @@ nilfs_mdt_write_page(struct page *page, struct writeback_control *wbc)
         struct inode *inode = container_of(page->mapping,
                                            struct inode, i_data);
         struct super_block *sb = inode->i_sb;
+        struct the_nilfs *nilfs = NILFS_MDT(inode)->mi_nilfs;
         struct nilfs_sb_info *writer = NULL;
         int err = 0;
 
@@ -411,9 +412,10 @@ nilfs_mdt_write_page(struct page *page, struct writeback_control *wbc)
         if (page->mapping->assoc_mapping)
                 return 0; /* Do not request flush for shadow page cache */
         if (!sb) {
-                writer = nilfs_get_writer(NILFS_MDT(inode)->mi_nilfs);
+                down_read(&nilfs->ns_writer_sem);
+                writer = nilfs->ns_writer;
                 if (!writer) {
-                        nilfs_put_writer(NILFS_MDT(inode)->mi_nilfs);
+                        up_read(&nilfs->ns_writer_sem);
                         return -EROFS;
                 }
                 sb = writer->s_super;
@@ -425,7 +427,7 @@ nilfs_mdt_write_page(struct page *page, struct writeback_control *wbc)
                 nilfs_flush_segment(sb, inode->i_ino);
 
         if (writer)
-                nilfs_put_writer(NILFS_MDT(inode)->mi_nilfs);
+                up_read(&nilfs->ns_writer_sem);
         return err;
 }
 
diff --git a/fs/nilfs2/the_nilfs.c b/fs/nilfs2/the_nilfs.c
index 8b8889825716..0c4573653b87 100644
--- a/fs/nilfs2/the_nilfs.c
+++ b/fs/nilfs2/the_nilfs.c
@@ -68,12 +68,11 @@ static struct the_nilfs *alloc_nilfs(struct block_device *bdev)
 
         nilfs->ns_bdev = bdev;
         atomic_set(&nilfs->ns_count, 1);
-        atomic_set(&nilfs->ns_writer_refcount, -1);
         atomic_set(&nilfs->ns_ndirtyblks, 0);
         init_rwsem(&nilfs->ns_sem);
         init_rwsem(&nilfs->ns_super_sem);
         mutex_init(&nilfs->ns_mount_mutex);
-        mutex_init(&nilfs->ns_writer_mutex);
+        init_rwsem(&nilfs->ns_writer_sem);
         INIT_LIST_HEAD(&nilfs->ns_list);
         INIT_LIST_HEAD(&nilfs->ns_supers);
         spin_lock_init(&nilfs->ns_last_segment_lock);
diff --git a/fs/nilfs2/the_nilfs.h b/fs/nilfs2/the_nilfs.h
index 1b9caafb8662..fa3a1dfe4053 100644
--- a/fs/nilfs2/the_nilfs.h
+++ b/fs/nilfs2/the_nilfs.h
@@ -50,8 +50,7 @@ enum {
  * @ns_sem: semaphore for shared states
  * @ns_super_sem: semaphore for global operations across super block instances
  * @ns_mount_mutex: mutex protecting mount process of nilfs
- * @ns_writer_mutex: mutex protecting ns_writer attach/detach
- * @ns_writer_refcount: number of referrers on ns_writer
+ * @ns_writer_sem: semaphore protecting ns_writer attach/detach
  * @ns_current: back pointer to current mount
  * @ns_sbh: buffer heads of on-disk super blocks
  * @ns_sbp: pointers to super block data
@@ -100,8 +99,7 @@ struct the_nilfs {
         struct rw_semaphore     ns_sem;
         struct rw_semaphore     ns_super_sem;
         struct mutex            ns_mount_mutex;
-        struct mutex            ns_writer_mutex;
-        atomic_t                ns_writer_refcount;
+        struct rw_semaphore     ns_writer_sem;
 
         /*
          * components protected by ns_super_sem
@@ -221,34 +219,21 @@ static inline void get_nilfs(struct the_nilfs *nilfs)
         atomic_inc(&nilfs->ns_count);
 }
 
-static inline struct nilfs_sb_info *nilfs_get_writer(struct the_nilfs *nilfs)
-{
-        if (atomic_inc_and_test(&nilfs->ns_writer_refcount))
-                mutex_lock(&nilfs->ns_writer_mutex);
-        return nilfs->ns_writer;
-}
-
-static inline void nilfs_put_writer(struct the_nilfs *nilfs)
-{
-        if (atomic_add_negative(-1, &nilfs->ns_writer_refcount))
-                mutex_unlock(&nilfs->ns_writer_mutex);
-}
-
 static inline void
 nilfs_attach_writer(struct the_nilfs *nilfs, struct nilfs_sb_info *sbi)
 {
-        mutex_lock(&nilfs->ns_writer_mutex);
+        down_write(&nilfs->ns_writer_sem);
         nilfs->ns_writer = sbi;
-        mutex_unlock(&nilfs->ns_writer_mutex);
+        up_write(&nilfs->ns_writer_sem);
 }
 
 static inline void
 nilfs_detach_writer(struct the_nilfs *nilfs, struct nilfs_sb_info *sbi)
 {
-        mutex_lock(&nilfs->ns_writer_mutex);
+        down_write(&nilfs->ns_writer_sem);
         if (sbi == nilfs->ns_writer)
                 nilfs->ns_writer = NULL;
-        mutex_unlock(&nilfs->ns_writer_mutex);
+        up_write(&nilfs->ns_writer_sem);
 }
 
 static inline void nilfs_put_sbinfo(struct nilfs_sb_info *sbi)