[PATCH 2/2 v2] signal: Don't disable preemption in ptrace_stop() on PREEMPT_RT.

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On PREEMPT_RT keeping preemption disabled during the invocation of
cgroup_enter_frozen() is a problem because the function acquires css_set_lock
which is a sleeping lock on PREEMPT_RT and must not be acquired with disabled
preemption.
The preempt-disabled section is only for performance optimisation
reasons and can be avoided.

Extend the comment and don't disable preemption before scheduling on
PREEMPT_RT.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---

Is this better?

v1…v2:
  - Extend the comment to note that preemption isn't disabled due to
    the lock to make it obvious that the optimisation isn't just
    harmful but also pointless.

 kernel/signal.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index da017a5461163..dcb0b1fbcb3a8 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2328,11 +2328,20 @@ static int ptrace_stop(int exit_code, int why, unsigned long message,
 	 * The preempt-disable section ensures that there will be no preemption
 	 * between unlock and schedule() and so improving the performance since
 	 * the ptracer has no reason to sleep.
+	 *
+	 * On PREEMPT_RT locking tasklist_lock does not disable preemption.
+	 * Therefore the task can be preempted (after
+	 * do_notify_parent_cldstop()) before unlocking tasklist_lock so there
+	 * is no benefit in doing this. The optimisation is harmful on
+	 * PEEMPT_RT because the spinlock_t (in cgroup_enter_frozen()) must not
+	 * be acquired with disabled preemption.
 	 */
-	preempt_disable();
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_disable();
 	read_unlock(&tasklist_lock);
 	cgroup_enter_frozen();
-	preempt_enable_no_resched();
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_enable_no_resched();
 	schedule();
 	cgroup_leave_frozen(true);
 
-- 
2.40.1