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From: "Eric W. Biederman" <ebiederm@xmission.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	linux-kernel@vger.kernel.org, Ben Segall <bsegall@google.com>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Ingo Molnar <mingo@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>, Mel Gorman <mgorman@suse.de>,
	Steven Rostedt <rostedt@goodmis.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Vincent Guittot <vincent.guittot@linaro.org>
Subject: Re: [PATCH v2] ptrace: fix ptrace vs tasklist_lock race on PREEMPT_RT.
Date: Mon, 11 Apr 2022 08:44:24 -0500	[thread overview]
Message-ID: <87ee23zqdj.fsf@email.froward.int.ebiederm.org> (raw)
In-Reply-To: <YlQSe6m2tidxwHt0@hirez.programming.kicks-ass.net> (Peter Zijlstra's message of "Mon, 11 Apr 2022 13:35:23 +0200")

Peter Zijlstra <peterz@infradead.org> writes:

> On Fri, Apr 08, 2022 at 10:06:30PM +0200, Peter Zijlstra wrote:
>
>> I'll ponder if wait_task_inactive() can simplify things..
>
> This,.. so ptrace_check_attach(), which does ptrace_freeze_traced()
> already does wait_task_inactive(), but on the 'wrong' side of things.
>
> AFAICT, if we move that up, we're almost there, except that opens up a
> detach+attach race. That could be fixed by doing another
> wait_task_inactive(), but we can't due to locking :/
>
> Let's see if I can make that work without making a mess of things.
> Because ensuring the task is stuck in schedule() makes the whole
> saved_state thing go away -- as you noted.

The code can perhaps synchronize on a bit using the the full locking and
then drop the locks and call the wait_task_inactive or whatever.

The challenge as I see it is after the traced task is inactive to allow
"wake_up_state(t, TASK_WAKEKILL)" on the traced task, have the traced
tasks state change to TASK_RUNNING and not allow the traced task to run
until what is today ptrace_unfreeze_task is called.

I just don't know how to get something stuck and not allow it to run.

Eric

  reply	other threads:[~2022-04-11 13:47 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-03-02 21:04 [PATCH] ptrace: fix ptrace vs tasklist_lock race on PREEMPT_RT Sebastian Andrzej Siewior
2022-03-14  9:27 ` Sebastian Andrzej Siewior
2022-03-14 18:54 ` Oleg Nesterov
2022-03-15  8:31   ` Sebastian Andrzej Siewior
2022-03-15 14:29     ` Oleg Nesterov
2022-03-16  8:23       ` Sebastian Andrzej Siewior
2022-03-31 14:25       ` [PATCH v2] " Sebastian Andrzej Siewior
2022-04-04 16:13         ` Oleg Nesterov
2022-04-05  8:34           ` Oleg Nesterov
2022-04-05 10:10         ` Peter Zijlstra
2022-04-05 10:29           ` Oleg Nesterov
2022-04-05 11:28             ` Peter Zijlstra
2022-04-07 12:13               ` Peter Zijlstra
2022-04-07 17:51                 ` Eric W. Biederman
2022-04-07 22:50                 ` Eric W. Biederman
2022-04-08  9:09                   ` Peter Zijlstra
2022-04-08 19:40                     ` Eric W. Biederman
2022-04-08 20:06                       ` Peter Zijlstra
2022-04-11 11:35                         ` Peter Zijlstra
2022-04-11 13:44                           ` Eric W. Biederman [this message]
2022-04-11 17:07                             ` Peter Zijlstra
2022-04-12 11:59                               ` Peter Zijlstra

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