nilfs2: fix lock order reversal in chcp operation

Will fix the following lock order reversal lockdep detected:

=======================================================
[ INFO: possible circular locking dependency detected ]
2.6.32-rc6 #7
-------------------------------------------------------
chcp/30157 is trying to acquire lock:
 (&nilfs->ns_mount_mutex){+.+.+.}, at: [<fed7cfcc>] nilfs_cpfile_change_cpmode+0x46/0x752 [nilfs2]

but task is already holding lock:
 (&nilfs->ns_segctor_sem){++++.+}, at: [<fed7ca32>] nilfs_transaction_begin+0xba/0x110 [nilfs2]

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #2 (&nilfs->ns_segctor_sem){++++.+}:
       [<c105799c>] __lock_acquire+0x109c/0x139d
       [<c1057d26>] lock_acquire+0x89/0xa0
       [<c14151e2>] down_read+0x31/0x45
       [<fed6d77b>] nilfs_attach_checkpoint+0x8f/0x16b [nilfs2]
       [<fed6e393>] nilfs_get_sb+0x3e7/0x653 [nilfs2]
       [<c10c0ccb>] vfs_kern_mount+0x8b/0x124
       [<c10c0db2>] do_kern_mount+0x37/0xc3
       [<c10d7517>] do_mount+0x64d/0x69d
       [<c10d75cd>] sys_mount+0x66/0x95
       [<c1002a14>] sysenter_do_call+0x12/0x32

-> #1 (&type->s_umount_key#31/1){+.+.+.}:
       [<c105799c>] __lock_acquire+0x109c/0x139d
       [<c1057d26>] lock_acquire+0x89/0xa0
       [<c104c0f3>] down_write_nested+0x34/0x52
       [<c10c08fe>] sget+0x22e/0x389
       [<fed6e133>] nilfs_get_sb+0x187/0x653 [nilfs2]
       [<c10c0ccb>] vfs_kern_mount+0x8b/0x124
       [<c10c0db2>] do_kern_mount+0x37/0xc3
       [<c10d7517>] do_mount+0x64d/0x69d
       [<c10d75cd>] sys_mount+0x66/0x95
       [<c1002a14>] sysenter_do_call+0x12/0x32

-> #0 (&nilfs->ns_mount_mutex){+.+.+.}:
       [<c1057727>] __lock_acquire+0xe27/0x139d
       [<c1057d26>] lock_acquire+0x89/0xa0
       [<c1414d63>] mutex_lock_nested+0x41/0x23e
       [<fed7cfcc>] nilfs_cpfile_change_cpmode+0x46/0x752 [nilfs2]
       [<fed801b2>] nilfs_ioctl+0x11a/0x7da [nilfs2]
       [<c10cca12>] vfs_ioctl+0x27/0x6e
       [<c10ccf93>] do_vfs_ioctl+0x491/0x4db
       [<c10cd022>] sys_ioctl+0x45/0x5f
       [<c1002a14>] sysenter_do_call+0x12/0x32

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

2 files changed, 5 insertions, 3 deletions

diff --git a/fs/nilfs2/cpfile.c b/fs/nilfs2/cpfile.c
index 1c6cfb59128d..3f5d5d06f53c 100644
--- a/fs/nilfs2/cpfile.c
+++ b/fs/nilfs2/cpfile.c
@@ -871,7 +871,6 @@ int nilfs_cpfile_change_cpmode(struct inode *cpfile, __u64 cno, int mode)
                  * exclusive with a new mount job.  Though it doesn't cover
                  * umount, it's enough for the purpose.
                  */
-                mutex_lock(&nilfs->ns_mount_mutex);
                 if (nilfs_checkpoint_is_mounted(nilfs, cno, 1)) {
                         /* Current implementation does not have to protect
                            plain read-only mounts since they are exclusive
@@ -880,7 +879,6 @@ int nilfs_cpfile_change_cpmode(struct inode *cpfile, __u64 cno, int mode)
                         ret = -EBUSY;
                 } else
                         ret = nilfs_cpfile_clear_snapshot(cpfile, cno);
-                mutex_unlock(&nilfs->ns_mount_mutex);
                 return ret;
         case NILFS_SNAPSHOT:
                 return nilfs_cpfile_set_snapshot(cpfile, cno);
diff --git a/fs/nilfs2/ioctl.c b/fs/nilfs2/ioctl.c
index d24057d58f17..f6af76042d80 100644
--- a/fs/nilfs2/ioctl.c
+++ b/fs/nilfs2/ioctl.c
@@ -99,7 +99,8 @@ static int nilfs_ioctl_wrap_copy(struct the_nilfs *nilfs,
 static int nilfs_ioctl_change_cpmode(struct inode *inode, struct file *filp,
                                      unsigned int cmd, void __user *argp)
 {
-        struct inode *cpfile = NILFS_SB(inode->i_sb)->s_nilfs->ns_cpfile;
+        struct the_nilfs *nilfs = NILFS_SB(inode->i_sb)->s_nilfs;
+        struct inode *cpfile = nilfs->ns_cpfile;
         struct nilfs_transaction_info ti;
         struct nilfs_cpmode cpmode;
         int ret;
@@ -109,14 +110,17 @@ static int nilfs_ioctl_change_cpmode(struct inode *inode, struct file *filp,
         if (copy_from_user(&cpmode, argp, sizeof(cpmode)))
                 return -EFAULT;
 
+        mutex_lock(&nilfs->ns_mount_mutex);
         nilfs_transaction_begin(inode->i_sb, &ti, 0);
         ret = nilfs_cpfile_change_cpmode(
                 cpfile, cpmode.cm_cno, cpmode.cm_mode);
         if (unlikely(ret < 0)) {
                 nilfs_transaction_abort(inode->i_sb);
+                mutex_unlock(&nilfs->ns_mount_mutex);
                 return ret;
         }
         nilfs_transaction_commit(inode->i_sb); /* never fails */
+        mutex_unlock(&nilfs->ns_mount_mutex);
         return ret;
 }