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-rw-r--r--debian/patches-rt/0002-signal-Don-t-disable-preemption-in-ptrace_stop-on-PR.patch66
1 files changed, 66 insertions, 0 deletions
diff --git a/debian/patches-rt/0002-signal-Don-t-disable-preemption-in-ptrace_stop-on-PR.patch b/debian/patches-rt/0002-signal-Don-t-disable-preemption-in-ptrace_stop-on-PR.patch
new file mode 100644
index 000000000..05ec116ad
--- /dev/null
+++ b/debian/patches-rt/0002-signal-Don-t-disable-preemption-in-ptrace_stop-on-PR.patch
@@ -0,0 +1,66 @@
+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
+