linux-kernel - Re: [tip: core/core] signal: Add a proper comment about preempt_disable() in ptrace

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Date:   Wed, 20 Sep 2023 08:57:00 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     linux-kernel@...r.kernel.org
Cc:     linux-tip-commits@...r.kernel.org,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Oleg Nesterov <oleg@...hat.com>, x86@...nel.org
Subject: Re: [tip: core/core] signal: Add a proper comment about
 preempt_disable() in ptrace_stop()


* tip-bot2 for Sebastian Andrzej Siewior <tip-bot2@...utronix.de> wrote:

> 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@...utronix.de>
> AuthorDate:    Thu, 03 Aug 2023 12:09:31 +02:00
> Committer:     Thomas Gleixner <tglx@...utronix.de>
> 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@...utronix.de>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Acked-by: Oleg Nesterov <oleg@...hat.com>
> 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 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);
>  

Minor speling nits:

>  	/*
> -	 * 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

s/One
 /On

> +	 * 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

s/improving the performance
 /improving performance

> +	 * the tracee is scheduled out once it gets on the CPU.
>  	 */
>  	preempt_disable();
>  	read_unlock(&tasklist_lock);

Thanks,

	Ingo