[PATCH] NFS4: fix for recursive locking problem

When I was performing some operations on NFS, I got below error on server
side.

=============================================
[ INFO: possible recursive locking detected ]
2.6.19-prep #1
---------------------------------------------
nfsd4/3525 is trying to acquire lock:
(&inode->i_mutex){--..}, at: [<c0611e5a>] mutex_lock+0x21/0x24

but task is already holding lock:
(&inode->i_mutex){--..}, at: [<c0611e5a>] mutex_lock+0x21/0x24

other info that might help us debug this:
2 locks held by nfsd4/3525:
#0: (client_mutex){--..}, at: [<c0611e5a>] mutex_lock+0x21/0x24
#1: (&inode->i_mutex){--..}, at: [<c0611e5a>] mutex_lock+0x21/0x24

stack backtrace:
[<c04051ed>] show_trace_log_lvl+0x58/0x16a
[<c04057fa>] show_trace+0xd/0x10
[<c0405913>] dump_stack+0x19/0x1b
[<c043b6f1>] __lock_acquire+0x778/0x99c
[<c043be86>] lock_acquire+0x4b/0x6d
[<c0611ceb>] __mutex_lock_slowpath+0xbc/0x20a
[<c0611e5a>] mutex_lock+0x21/0x24
[<c047fd7e>] vfs_rmdir+0x76/0xf8
[<f94b7ce9>] nfsd4_clear_clid_dir+0x2c/0x41 [nfsd]
[<f94b7de9>] nfsd4_remove_clid_dir+0xb1/0xe8 [nfsd]
[<f94b307b>] laundromat_main+0x9b/0x1c3 [nfsd]
[<c04333d6>] run_workqueue+0x7a/0xbb
[<c0433d0b>] worker_thread+0xd2/0x107
[<c0436285>] kthread+0xc3/0xf2
[<c0402005>] kernel_thread_helper+0x5/0xb
===================================================================

Cause for this problem was,2 successive mutex_lock calls on 2 diffrent inodes ,as shown below

static int
nfsd4_clear_clid_dir(struct dentry *dir, struct dentry *dentry)
{
int status;

/* For now this directory should already be empty, but we empty it of
* any regular files anyway, just in case the directory was created by
* a kernel from the future.... */
nfsd4_list_rec_dir(dentry, nfsd4_remove_clid_file);
mutex_lock(&dir->d_inode->i_mutex);
status = vfs_rmdir(dir->d_inode, dentry);
...

int vfs_rmdir(struct inode *dir, struct dentry *dentry)
{
int error = may_delete(dir, dentry, 1);

if (error)
return error;

if (!dir->i_op || !dir->i_op->rmdir)
return -EPERM;

DQUOT_INIT(dir);

mutex_lock(&dentry->d_inode->i_mutex);
...

So I have developed the patch to overcome this problem.

Signed-off-by: Srinivasa DS <[e-mail]>
Cc: Neil Brown <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/nfsd/nfs4recover.c

···
 	 * any regular files anyway, just in case the directory was created by
 	 * a kernel from the future.... */
 	nfsd4_list_rec_dir(dentry, nfsd4_remove_clid_file);
-	mutex_lock(&dir->d_inode->i_mutex);
+	mutex_lock_nested(&dir->d_inode->i_mutex, I_MUTEX_PARENT);
 	status = vfs_rmdir(dir->d_inode, dentry);
 	mutex_unlock(&dir->d_inode->i_mutex);
 	return status;