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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>, rjw@rjwysocki.net,
        mingo@kernel.org, vincent.guittot@linaro.org,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, mgorman@suse.de,
        bigeasy@linutronix.de, Will Deacon <will@kernel.org>,
        linux-kernel@vger.kernel.org, tj@kernel.org,
        linux-pm@vger.kernel.org
In-Reply-To: <20220426055122.GA29684@redhat.com> (Oleg Nesterov's message of
        "Tue, 26 Apr 2022 07:51:23 +0200")
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Date:   Tue, 26 Apr 2022 12:19:20 -0500
Message-ID: <878rrrkbjb.fsf@email.froward.int.ebiederm.org>
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Subject: Re: [PATCH v2 2/5] sched,ptrace: Fix ptrace_check_attach() vs
 PREEMPT_RT
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Oleg Nesterov <oleg@redhat.com> writes:

> On 04/25, Eric W. Biederman wrote:
>>
>> I don't see JOBCTL_TRACED_QUIESCE being cleared "if (!current->ptrace)".
>
> As Peter explained, in this case we can rely on __ptrace_unlink() which
> should clear this flag.

I had missed that that signal_wake_up_state was clearing
JOBCTL_TRACED_QUIESCE.

Relying on __ptrace_unlink assumes the __ptrace_unlink happens after
siglock is taken before calling ptrace_stop.  Especially with the
ptrace_notify in signal_delivered that does not look guaranteed.

The __ptrace_unlink could also happen during arch_ptrace_stop.

Relying on siglock is sufficient because __ptrace_unlink holds siglock
over clearing task->ptrace.  Which means that the simple fix for this is
to just test task->ptrace before we set JOBCTL_TRACED_QUEIESCE.

Eric