[PATCH] s390/debug: Fix deadlock during unregister

[PATCH] s390/debug: Fix deadlock during unregister

[Peter Oberparleiter]

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

Re: [PATCH] s390/debug: Fix deadlock during unregister

[sashiko-bot]

> 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>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260812075319.2007940-1-oberpar@linux.ibm.com?part=1

Re: [PATCH] s390/debug: Fix deadlock during unregister

[Heiko Carstens]

On Wed, Aug 12, 2026 at 09:53:19AM +0200, Peter Oberparleiter wrote:
> 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(-)

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>

Re: [PATCH] s390/debug: Fix deadlock during unregister

[Vasily Gorbik]

On Wed, Aug 12, 2026 at 09:53:19AM +0200, Peter Oberparleiter wrote:
> 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(-)

Added
Fixes: 9372a82892c2 ("s390/debug: fix debug area life cycle")

and applied, thank you!