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Date:   Thu, 28 Apr 2022 20:22:58 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        linux-kernel@vger.kernel.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>, tj@kernel.org,
        linux-pm@vger.kernel.org, Richard Weinberger <richard@nod.at>,
        Anton Ivanov <anton.ivanov@cambridgegreys.com>,
        Johannes Berg <johannes@sipsolutions.net>,
        linux-um@lists.infradead.org, Chris Zankel <chris@zankel.net>,
        Max Filippov <jcmvbkbc@gmail.com>,
        inux-xtensa@linux-xtensa.org, Kees Cook <keescook@chromium.org>,
        Jann Horn <jannh@google.com>
Subject: Re: [PATCH 6/9] signal: Always call do_notify_parent_cldstop with
 siglock held
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On 04/28, Peter Zijlstra wrote:
>
> I've not had time to fully appreciate the nested locking here, but if it
> is possible to rework things to always take both locks at the same time,
> then it would be possible to impose an arbitrary lock order on things
> and break the cycle that way.

This is clear, but this is not that simple.

For example (with this series at least), ptrace_stop() already holds
current->sighand->siglock which (in particular) we need to protect
current->parent, but then we need current->parent->sighand->siglock
in do_notify_parent_cldstop().

Oleg.