[PATCH] s390/debug: Fix deadlock during unregister

[Peter Oberparleiter <oberpar@linux.ibm.com>]

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.

Signed-off-by: Peter Oberparleiter <oberpar@linux.ibm.com>
---
 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 dbf430f479bd..14d2b58ad093 100644
--- a/arch/s390/kernel/debug.c
+++ b/arch/s390/kernel/debug.c
@@ -993,8 +993,8 @@ 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;
 
@@ -1004,6 +1004,11 @@ 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)
@@ -1029,6 +1034,7 @@ 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