[PATCH 6.1 0998/1191] s390/debug: Fix deadlock during unregister

6.1-stable review patch.  If anyone has any objections, please let me know.

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

From: Peter Oberparleiter <oberpar@linux.ibm.com>

[ Upstream commit 445c31ac638fd1af203d79bdf25fc0cb3149fbbc ]

Unregistering an s390dbf debug area while one of the associated debugfs
files is being written to can cause a deadlock:

$ echo >.../vmur/level    $ rmmod vmur
===================================================
debugfs write
debugfs_file_get()
                          debug_unregister()
                          mutex_lock(debug_mutex)
                          debugfs_remove()
                          wait for debugfs_file_put()
debug_file_ops.write()
debug_input()
mutex_lock(debug_mutex) ==> DEADLOCK

Fix this by splitting debug_unregister() into an s390dbf and debugfs
part, and running only the s390dbf part with debug_mutex locked.

Fixes: 9372a82892c2 ("s390/debug: fix debug area life cycle")
Signed-off-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/s390/kernel/debug.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/s390/kernel/debug.c b/arch/s390/kernel/debug.c
index 65302aea77e85..8cc747b39c84d 100644
--- a/arch/s390/kernel/debug.c
+++ b/arch/s390/kernel/debug.c

@@ -747,8 +747,8 @@ void debug_register_static(debug_info_t *id, int pages_per_area, int nr_areas)
 	mutex_unlock(&debug_mutex);
 }
 
-/* Remove debugfs entries and remove from internal list. */
-static void _debug_unregister(debug_info_t *id)
+/* Remove debugfs entries. */
+static void _debug_unregister_debugfs(debug_info_t *id)
 {
 	int i;
 

@@ -758,6 +758,11 @@ static void _debug_unregister(debug_info_t *id)
 		debugfs_remove(id->debugfs_entries[i]);
 	}
 	debugfs_remove(id->debugfs_root_entry);
+}
+
+/* Remove from internal list. */
+static void _debug_unregister(debug_info_t *id)
+{
 	if (id == debug_area_first)
 		debug_area_first = id->next;
 	if (id == debug_area_last)

@@ -783,6 +788,7 @@ void debug_unregister(debug_info_t *id)
 	mutex_lock(&debug_mutex);
 	_debug_unregister(id);
 	mutex_unlock(&debug_mutex);
+	_debug_unregister_debugfs(id);
 
 	debug_info_put(id);
 }

-- 
2.53.0