linux-kernel - Re: [PATCH] signal: Revert ptrace preempt magic

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Date:   Thu, 24 May 2018 11:25:14 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Miklos Szeredi <mszeredi@...e.cz>,
        Oleg Nesterov <oleg@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] signal: Revert ptrace preempt magic

Sebastian Andrzej Siewior <bigeasy@...utronix.de> writes:

> From: Thomas Gleixner <tglx@...utronix.de>
>
> Upstream commit '53da1d9456fe7f8 fix ptrace slowness' is nothing more
> than a bandaid around the ptrace design trainwreck. It's not a
> correctness issue, it's merily a cosmetic bandaid.

This patch comes with no justification or reason to remove the
``cosmetic bandaid''.  The description in 53da1d9456fe ("fix ptrace
slowness") is quite persuasive that there is a real world issue.

So while this may be a good idea to remove this.  I don't see any
description of testing to indicate this won't cause uml to regresssion.
Nor do I see any compelling reason except code tidiness to remove this.

As such until adequate descriptions can be provideded.

Nacked-by: "Eric W. Biederman" <ebiederm@...ssion.com>


> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  kernel/signal.c |    8 --------
>  1 file changed, 8 deletions(-)
>
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1876,15 +1876,7 @@ static void ptrace_stop(int exit_code, i
>  		if (gstop_done && ptrace_reparented(current))
>  			do_notify_parent_cldstop(current, false, why);
>  
> -		/*
> -		 * Don't want to allow preemption here, because
> -		 * sys_ptrace() needs this task to be inactive.
> -		 *
> -		 * XXX: implement read_unlock_no_resched().
> -		 */
> -		preempt_disable();
>  		read_unlock(&tasklist_lock);
> -		preempt_enable_no_resched();
>  		freezable_schedule();
>  	} else {
>  		/*