linux-kernel - Re: [PATCH v4 10/12] ptrace: Don't change _

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Date:   Tue, 10 May 2022 17:34:53 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org, rjw@...ysocki.net, mingo@...nel.org,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, mgorman@...e.de, bigeasy@...utronix.de,
        Will Deacon <will@...nel.org>, tj@...nel.org,
        linux-pm@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        linux-um@...ts.infradead.org, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        linux-xtensa@...ux-xtensa.org, Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, linux-ia64@...r.kernel.org
Subject: Re: [PATCH v4 10/12] ptrace: Don't change __state

On 05/10, Eric W. Biederman wrote:
>
> > But I still think that a lockless
> >
> > 	if (!(task->jobctl & JOBCTL_PTRACE_FROZEN))
> > 		return;
> >
> > check at the start of ptrace_unfreeze_traced() makes sense to avoid
> > lock_task_sighand() if possible.
> >
> > And ptrace_resume() can probably clear JOBCTL_PTRACE_FROZEN along with
> > JOBCTL_TRACED to make this optimization work better. The same for
> > ptrace_signal_wake_up().
>
> What do you have that suggests that taking siglock there is a problem?

Not necessarily a problem, but this optimization is free. If the tracee
was resumed, it can compete for siglock with debugger.

> What you propose will definitely work as an incremental change, and
> in an incremental change we can explain why doing the stupid simple
> thing is not good enough.

OK.

Oleg.