Subject: [PATCH 2/2] signal: Don't disable preemption in ptrace_stop() on PREEMPT_RT.
From: Sebastian Andrzej Siewior <[email-masked]>
Date: Thu, 30 Mar 2023 21:07:54 +0200

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 <[email-masked]>
Link: https://lore.kernel.org/[email-masked]
---
 kernel/signal.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

Index: linux-6.3.0-rt11/kernel/signal.c
===================================================================
--- linux-6.3.0-rt11.orig/kernel/signal.c
+++ linux-6.3.0-rt11/kernel/signal.c
@@ -2351,11 +2351,16 @@ static int ptrace_stop(int exit_code, in
 	 * 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.
+	 *
+	 * This optimisation is not doable on PREEMPT_RT due to the spinlock_t
+	 * within the preempt-disable section.
 	 */
-	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);