Subject: [PATCH] signal: Don't disable preemption in ptrace_stop() on PREEMPT_RT.
From: Sebastian Andrzej Siewior <[email-masked]>
Date: Wed, 22 Jun 2022 11:36:17 +0200

Commit
   53da1d9456fe7 ("fix ptrace slowness")

is just band aid around the problem.
The invocation of do_notify_parent_cldstop() wakes the parent and makes
it runnable. The scheduler then wants to replace this still running task
with the parent. With the read_lock() acquired this is not possible
because preemption is disabled and so this is deferred until read_unlock().
This scheduling point is undesired and is avoided by disabling preemption
around the unlock operation enabled again before the schedule() invocation
without a preemption point.
This is only undesired because the parent sleeps a cycle in
wait_task_inactive() until the traced task leaves the run-queue in
schedule(). It is not a correctness issue, it is just band aid to avoid the
visbile delay which sums up over multiple invocations.
The task can still be preempted if an interrupt occurs between
preempt_enable_no_resched() and freezable_schedule() because on the IRQ-exit
path of the interrupt scheduling _will_ happen. This is ignored since it does
not happen very often.

On PREEMPT_RT keeping preemption disabled during the invocation of
cgroup_enter_frozen() becomes 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.

Don't disable preemption on PREEMPT_RT. Remove the TODO regarding adding
read_unlock_no_resched() as there is no need for it and will cause harm.

Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
Link: https://lkml.kernel.org/r/[email-masked]
---
 kernel/signal.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2298,13 +2298,13 @@ static int ptrace_stop(int exit_code, in
 	/*
 	 * Don't want to allow preemption here, because
 	 * sys_ptrace() needs this task to be inactive.
-	 *
-	 * XXX: implement read_unlock_no_resched().
 	 */
-	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);