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-rw-r--r--debian/patches-rt/0001-signal-Add-proper-comment-about-the-preempt-disable-.patch47
1 files changed, 47 insertions, 0 deletions
diff --git a/debian/patches-rt/0001-signal-Add-proper-comment-about-the-preempt-disable-.patch b/debian/patches-rt/0001-signal-Add-proper-comment-about-the-preempt-disable-.patch
new file mode 100644
index 0000000000..e00261d790
--- /dev/null
+++ b/debian/patches-rt/0001-signal-Add-proper-comment-about-the-preempt-disable-.patch
@@ -0,0 +1,47 @@
+From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
+Date: Thu, 3 Aug 2023 12:09:31 +0200
+Subject: [PATCH 1/2] signal: Add proper comment about the preempt-disable in
+ ptrace_stop().
+Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.6/older/patches-6.6.7-rt18.tar.xz
+
+Commit 53da1d9456fe7 ("fix ptrace slowness") added a preempt-disable section
+between read_unlock() and the following schedule() invocation without
+explaining why it is needed.
+
+Replace the comment with an explanation why this is needed. Clarify that
+it is needed for correctness but for performance reasons.
+
+Acked-by: Oleg Nesterov <oleg@redhat.com>
+Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
+Link: https://lore.kernel.org/r/20230803100932.325870-2-bigeasy@linutronix.de
+---
+ kernel/signal.c | 17 ++++++++++++++---
+ 1 file changed, 14 insertions(+), 3 deletions(-)
+
+--- a/kernel/signal.c
++++ b/kernel/signal.c
+@@ -2329,10 +2329,21 @@ static int ptrace_stop(int exit_code, in
+ do_notify_parent_cldstop(current, false, why);
+
+ /*
+- * Don't want to allow preemption here, because
+- * sys_ptrace() needs this task to be inactive.
++ * The previous do_notify_parent_cldstop() invocation woke ptracer.
++ * One a PREEMPTION kernel this can result in preemption requirement
++ * which will be fulfilled after read_unlock() and the ptracer will be
++ * put on the CPU.
++ * The ptracer is in wait_task_inactive(, __TASK_TRACED) waiting for
++ * this task wait in schedule(). If this task gets preempted then it
++ * remains enqueued on the runqueue. The ptracer will observe this and
++ * then sleep for a delay of one HZ tick. In the meantime this task
++ * gets scheduled, enters schedule() and will wait for the ptracer.
+ *
+- * XXX: implement read_unlock_no_resched().
++ * This preemption point is not bad from correctness point of view but
++ * extends the runtime by one HZ tick time due to the ptracer's sleep.
++ * 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.
+ */
+ preempt_disable();
+ read_unlock(&tasklist_lock);