[PATCH 2/2] lockdep/selftests: Restore sched_rt_mutex state on PREEMPT_RT

[Karl Mehltretter <kmehltretter@gmail.com>]

The WW-mutex selftests deliberately exercise failing lock paths. On
PREEMPT_RT, some of those paths enter the RT-mutex scheduler helpers.

The change referenced by the Fixes tag made those helpers track RT-mutex
scheduling state in current->sched_rt_mutex. The bit is normally cleared by
the matching post-schedule helper, but some WW-mutex selftests disable
the runtime debug_locks flag before that happens. With debug_locks cleared,
lockdep_assert() does not evaluate the expression that clears the bit,
leaving stale state for the next testcase.

With CONFIG_PREEMPT_RT=y and CONFIG_DEBUG_LOCKING_API_SELFTESTS=y, that
stale state produces warnings such as:

  WARNING: kernel/sched/core.c:7557 at rt_mutex_pre_schedule+0x26/0x2d
  RIP: 0010:rt_mutex_pre_schedule+0x26/0x2d

Save and restore current->sched_rt_mutex around each testcase, matching the
existing PREEMPT_RT cleanup for task-local migration and RCU state.

Fixes: d14f9e930b90 ("locking/rtmutex: Use rt_mutex specific scheduler helpers")
Assisted-by: Codex:gpt-5
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 lib/locking-selftest.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/lib/locking-selftest.c b/lib/locking-selftest.c
index 34554e26d477..e7c3ccb23b58 100644
--- a/lib/locking-selftest.c
+++ b/lib/locking-selftest.c
@@ -1433,6 +1433,7 @@ static void dotest(void (*testcase_fn)(void), int expected, int lockclass_mask)
 #ifdef CONFIG_PREEMPT_RT
 	int saved_mgd_count = current->migration_disabled;
 	int saved_rcu_count = current->rcu_read_lock_nesting;
+	int saved_sched_rt_mutex = current->sched_rt_mutex;
 #endif
 
 	WARN_ON(irqs_disabled());
@@ -1469,6 +1470,8 @@ static void dotest(void (*testcase_fn)(void), int expected, int lockclass_mask)
 	preempt_count_set(saved_preempt_count);
 
 #ifdef CONFIG_PREEMPT_RT
+	current->sched_rt_mutex = saved_sched_rt_mutex;
+
 	while (current->migration_disabled > saved_mgd_count)
 		migrate_enable();
 
-- 
2.39.5 (Apple Git-154)