[tip: core/core] signal: Add a proper comment about preempt_disable() in ptrace_stop()

[tip-bot2 for Sebastian Andrzej Siewior]

The following commit has been merged into the core/core branch of tip:

Commit-ID:     a20d6f63dbfc176697886d7709312ad0a795648e
Gitweb:        https://git.kernel.org/tip/a20d6f63dbfc176697886d7709312ad0a795648e
Author:        Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
AuthorDate:    Thu, 03 Aug 2023 12:09:31 +02:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Tue, 19 Sep 2023 22:08:29 +02:00

signal: Add a proper comment about preempt_disable() in ptrace_stop()

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 existing contentless comment with a proper explanation to
clarify that it is not needed for correctness but for performance reasons.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20230803100932.325870-2-bigeasy@xxxxxxxxxxxxx

---
 kernel/signal.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 0901901..3035beb 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2329,10 +2329,22 @@ static int ptrace_stop(int exit_code, int why, unsigned long message,
 		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 a 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 will observe that
+	 * the tracee is scheduled out once it gets on the CPU.
 	 */
 	preempt_disable();
 	read_unlock(&tasklist_lock);