dcache: Fix locking bugs in backported "deal with deadlock in d_walk()"

Steven Rostedt reported:
> Porting -rt to the latest 3.2 stable tree I triggered this bug:
> 
> =====================================
> [ BUG: bad unlock balance detected! ]
> -------------------------------------
> rm/1638 is trying to release lock (rcu_read_lock) at:
> [<c04fde6c>] rcu_read_unlock+0x0/0x23
> but there are no more locks to release!
> 
> other info that might help us debug this:
> 2 locks held by rm/1638:
>  #0:  (&sb->s_type->i_mutex_key#9/1){+.+.+.}, at: [<c04f93eb>] do_rmdir+0x5f/0xd2
>  #1:  (&sb->s_type->i_mutex_key#9){+.+.+.}, at: [<c04f9329>] vfs_rmdir+0x49/0xac
> 
> stack backtrace:
> Pid: 1638, comm: rm Not tainted 3.2.66-test-rt96+ #2
> Call Trace:
>  [<c083f390>] ? printk+0x1d/0x1f
>  [<c0463cdf>] print_unlock_inbalance_bug+0xc3/0xcd
>  [<c04653a8>] lock_release_non_nested+0x98/0x1ec
>  [<c046228d>] ? trace_hardirqs_off_caller+0x18/0x90
>  [<c0456f1c>] ? local_clock+0x2d/0x50
>  [<c04fde6c>] ? d_hash+0x2f/0x2f
>  [<c04fde6c>] ? d_hash+0x2f/0x2f
>  [<c046568e>] lock_release+0x192/0x1ad
>  [<c04fde83>] rcu_read_unlock+0x17/0x23
>  [<c04ff344>] shrink_dcache_parent+0x227/0x270
>  [<c04f9348>] vfs_rmdir+0x68/0xac
>  [<c04f9424>] do_rmdir+0x98/0xd2
>  [<c04f03ad>] ? fput+0x1a3/0x1ab
>  [<c084dd42>] ? sysenter_exit+0xf/0x1a
>  [<c0465b58>] ? trace_hardirqs_on_caller+0x118/0x149
>  [<c04fa3e0>] sys_unlinkat+0x2b/0x35
>  [<c084dd13>] sysenter_do_call+0x12/0x12
> 
> 
> 
> 
> There's a path to calling rcu_read_unlock() without calling
> rcu_read_lock() in have_submounts().
> 
> 	goto positive;
> 
> positive:
> 	if (!locked && read_seqretry(&rename_lock, seq))
> 		goto rename_retry;
> 
> rename_retry:
> 	rcu_read_unlock();
> 
> in the above path, rcu_read_lock() is never done before calling
> rcu_read_unlock();

I reviewed locking contexts in all three functions that I changed when
backporting "deal with deadlock in d_walk()".  It's actually worse
than this:

- We don't hold this_parent->d_lock at the 'positive' label in
  have_submounts(), but it is unlocked after 'rename_retry'.
- There is an rcu_read_unlock() after the 'out' label in
  select_parent(), but it's not held at the 'goto out'.

Fix all three lock imbalances.
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Tested-by: Steven Rostedt <rostedt@goodmis.org>

fs/dcache.c

@@ -1035,7 +1035,7 @@ int have_submounts(struct dentry *parent)
 	return 0; /* No mount points found in tree */
 positive:
 	if (!locked && read_seqretry(&rename_lock, seq))
-		goto rename_retry;
+		goto rename_retry_unlocked;
 	if (locked)
 		write_sequnlock(&rename_lock);
 	return 1;
@@ -1045,6 +1045,7 @@ int have_submounts(struct dentry *parent)
 	rcu_read_unlock();
 	if (locked)
 		goto again;
+rename_retry_unlocked:
 	locked = 1;
 	write_seqlock(&rename_lock);
 	goto again;
@@ -1109,6 +1110,7 @@ static int select_parent(struct dentry *parent, struct list_head *dispose)
 		 */
 		if (found && need_resched()) {
 			spin_unlock(&dentry->d_lock);
+			rcu_read_lock();
 			goto out;
 		}