From a20d6f63dbfc176697886d7709312ad0a795648e Mon Sep 17 00:00:00 2001 From: Sebastian Andrzej Siewior Date: Thu, 3 Aug 2023 12:09:31 +0200 Subject: [PATCH] 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 Signed-off-by: Thomas Gleixner Acked-by: Oleg Nesterov Link: https://lore.kernel.org/r/20230803100932.325870-2-bigeasy@linutronix.de --- kernel/signal.c | 18 +++++++++++++++--- 1 file changed, 15 insertions(+), 3 deletions(-) diff --git a/kernel/signal.c b/kernel/signal.c index 09019017d6690..3035bebd7075e 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); -- 2.30.2