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From d5541b6ef4eccee650abfe3095b9e7365773494c Mon Sep 17 00:00:00 2001
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Wed, 22 Jun 2022 11:36:17 +0200
Subject: [PATCH 02/62] signal: Don't disable preemption in ptrace_stop() on
PREEMPT_RT.
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.1/older/patches-6.1.69-rt21.tar.xz
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 <bigeasy@linutronix.de>
Link: https://lkml.kernel.org/r/20220720154435.232749-2-bigeasy@linutronix.de
---
kernel/signal.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)
diff --git a/kernel/signal.c b/kernel/signal.c
index 5d45f5da2b36..58e919c7c936 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2302,13 +2302,13 @@ static int ptrace_stop(int exit_code, int why, unsigned long message,
/*
* 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);
--
2.43.0
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