linux-kernel - [PATCH 3.16 173/204] ALSA: seq: Avoid invalid lockdep class warning

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Date:   Thu, 28 Dec 2017 17:05:44 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Takashi Iwai" <tiwai@...e.de>,
        "syzbot" <syzkaller@...glegroups.com>
Subject: [PATCH 3.16 173/204] ALSA: seq: Avoid invalid lockdep class warning

3.16.52-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Takashi Iwai <tiwai@...e.de>

commit 3510c7aa069aa83a2de6dab2b41401a198317bdc upstream.

The recent fix for adding rwsem nesting annotation was using the given
"hop" argument as the lock subclass key.  Although the idea itself
works, it may trigger a kernel warning like:
  BUG: looking up invalid subclass: 8
  ....
since the lockdep has a smaller number of subclasses (8) than we
currently allow for the hops there (10).

The current definition is merely a sanity check for avoiding the too
deep delivery paths, and the 8 hops are already enough.  So, as a
quick fix, just follow the max hops as same as the max lockdep
subclasses.

Fixes: 1f20f9ff57ca ("ALSA: seq: Fix nested rwsem annotation for lockdep splat")
Reported-by: syzbot <syzkaller@...glegroups.com>
Signed-off-by: Takashi Iwai <tiwai@...e.de>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 include/sound/seq_kernel.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/include/sound/seq_kernel.h
+++ b/include/sound/seq_kernel.h
@@ -55,7 +55,8 @@ typedef union snd_seq_timestamp snd_seq_
 #define SNDRV_SEQ_DEFAULT_CLIENT_EVENTS	200

 /* max delivery path length */
-#define SNDRV_SEQ_MAX_HOPS		10
+/* NOTE: this shouldn't be greater than MAX_LOCKDEP_SUBCLASSES */
+#define SNDRV_SEQ_MAX_HOPS		8

 /* max size of event size */
 #define SNDRV_SEQ_MAX_EVENT_LEN		0x3fffffff