oprofile, dcookies: Fix possible circular locking dependency

The lockdep warning below detects a possible A->B/B->A locking
dependency of mm->mmap_sem and dcookie_mutex. The order in
sync_buffer() is mm->mmap_sem/dcookie_mutex, while in
sys_lookup_dcookie() it is vice versa.

Fixing it in sys_lookup_dcookie() by unlocking dcookie_mutex before
copy_to_user().

oprofiled/4432 is trying to acquire lock:
(&mm->mmap_sem){++++++}, at: [<ffffffff810b444b>] might_fault+0x53/0xa3

but task is already holding lock:
(dcookie_mutex){+.+.+.}, at: [<ffffffff81124d28>] sys_lookup_dcookie+0x45/0x149

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (dcookie_mutex){+.+.+.}:
[<ffffffff8106557f>] lock_acquire+0xf8/0x11e
[<ffffffff814634f0>] mutex_lock_nested+0x63/0x309
[<ffffffff81124e5c>] get_dcookie+0x30/0x144
[<ffffffffa0000fba>] sync_buffer+0x196/0x3ec [oprofile]
[<ffffffffa0001226>] task_exit_notify+0x16/0x1a [oprofile]
[<ffffffff81467b96>] notifier_call_chain+0x37/0x63
[<ffffffff8105803d>] __blocking_notifier_call_chain+0x50/0x67
[<ffffffff81058068>] blocking_notifier_call_chain+0x14/0x16
[<ffffffff8105a718>] profile_task_exit+0x1a/0x1c
[<ffffffff81039e8f>] do_exit+0x2a/0x6fc
[<ffffffff8103a5e4>] do_group_exit+0x83/0xae
[<ffffffff8103a626>] sys_exit_group+0x17/0x1b
[<ffffffff8146ad4b>] system_call_fastpath+0x16/0x1b

-> #0 (&mm->mmap_sem){++++++}:
[<ffffffff81064dfb>] __lock_acquire+0x1085/0x1711
[<ffffffff8106557f>] lock_acquire+0xf8/0x11e
[<ffffffff810b4478>] might_fault+0x80/0xa3
[<ffffffff81124de7>] sys_lookup_dcookie+0x104/0x149
[<ffffffff8146ad4b>] system_call_fastpath+0x16/0x1b

other info that might help us debug this:

1 lock held by oprofiled/4432:
#0: (dcookie_mutex){+.+.+.}, at: [<ffffffff81124d28>] sys_lookup_dcookie+0x45/0x149

stack backtrace:
Pid: 4432, comm: oprofiled Not tainted 2.6.39-00008-ge5a450d #9
Call Trace:
[<ffffffff81063193>] print_circular_bug+0xae/0xbc
[<ffffffff81064dfb>] __lock_acquire+0x1085/0x1711
[<ffffffff8102ef13>] ? get_parent_ip+0x11/0x42
[<ffffffff810b444b>] ? might_fault+0x53/0xa3
[<ffffffff8106557f>] lock_acquire+0xf8/0x11e
[<ffffffff810b444b>] ? might_fault+0x53/0xa3
[<ffffffff810d7d54>] ? path_put+0x22/0x27
[<ffffffff810b4478>] might_fault+0x80/0xa3
[<ffffffff810b444b>] ? might_fault+0x53/0xa3
[<ffffffff81124de7>] sys_lookup_dcookie+0x104/0x149
[<ffffffff8146ad4b>] system_call_fastpath+0x16/0x1b

References: https://bugzilla.kernel.org/show_bug.cgi?id=13809
Cc: <[e-mail]> # .27+
Signed-off-by: Robert Richter <[e-mail]>

fs/dcookies.c

···
 	/* FIXME: (deleted) ? */
 	path = d_path(&dcs->path, kbuf, PAGE_SIZE);
 
+	mutex_unlock(&dcookie_mutex);
+
 	if (IS_ERR(path)) {
 		err = PTR_ERR(path);
 		goto out_free;
···
 
 out_free:
 	kfree(kbuf);
+	return err;
 out:
 	mutex_unlock(&dcookie_mutex);
 	return err;