signal: Don't disable preemption in ptrace_stop() on PREEMPT_RT

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>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20230803100932.325870-3-bigeasy@linutronix.de

diff --git a/kernel/signal.c b/kernel/signal.c
index 3035bebd7075e3926e96b53b8526743ece817263..f2a5578326adec96e3a7bb7b287fb60f001fcd07 100644 (file)
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2345,11 +2345,22 @@ static int ptrace_stop(int exit_code, int why, unsigned long message,
         * will be no preemption between unlock and schedule() and so
         * improving the performance since the ptracer will observe that
         * the tracee is scheduled out once it gets on the CPU.
+        *
+        * 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.
+        *
+        * In fact disabling preemption is harmful on PREEMPT_RT because
+        * the spinlock_t in cgroup_enter_frozen() must not be acquired
+        * with preemption disabled due to the 'sleeping' spinlock
+        * substitution of RT.
         */
-       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);