linux-kernel - Re: [PATCH v2 0/5] ptrace-vs-PREEMPT

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Date:   Fri, 22 Apr 2022 23:13:50 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, rjw@...ysocki.net,
        oleg@...hat.com, mingo@...nel.org, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, mgorman@...e.de,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        tj@...nel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH v2 0/5] ptrace-vs-PREEMPT_RT and freezer rewrite

On 2022-04-22 14:15:35 [-0500], Eric W. Biederman wrote:
> Sebastian Andrzej Siewior <bigeasy@...utronix.de> writes:
> 
> > On 2022-04-21 17:02:48 [+0200], Peter Zijlstra wrote:
> >> Find here a new posting of the ptrace and freezer patches :-)
> >> 
> >> The majority of the changes are in patch 2, which with much feedback from Oleg
> >> and Eric has changed lots.
> >> 
> >> I'm hoping we're converging on something agreeable.
> >
> > I tested this under RT (had to remove the preempt-disable section in
> > ptrace_stop()) with ssdd [0]. It forks a few tasks and then
> > PTRACE_SINGLESTEPs them for a few iterations.
> 
> Out of curiosity why did you need to remove the preempt_disable section
> on PREEMPT_RT?  It should have lasted for just a moment until schedule
> was called.

within that section spinlock_t locks are acquired. These locks are
sleeping locks on PREEMPT_RT and must not be acquired within a
preempt-disable section. (A spinlock_t lock does not disable preemption
on PREEMPT_RT.)

> Eric

Sebastian