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

[Sebastian Andrzej Siewior]

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@xxxxxxxxxxxxx>
---
 kernel/signal.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2352,11 +2352,16 @@ 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);