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Date:   Wed, 27 Apr 2022 16:10:25 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     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, Peter Zijlstra <peterz@infradead.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
Message-ID: <20220427141018.GA17421@redhat.com>
References: <878rrrh32q.fsf_-_@email.froward.int.ebiederm.org>
 <20220426225211.308418-6-ebiederm@xmission.com>
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On 04/26, Eric W. Biederman wrote:
>
> @@ -2164,7 +2166,9 @@ static void do_notify_parent_cldstop(struct task_struct *tsk,
>   	}
>
>  	sighand = parent->sighand;
> -	spin_lock_irqsave(&sighand->siglock, flags);
> +	lock = tsk->sighand != sighand;
> +	if (lock)
> +		spin_lock_nested(&sighand->siglock, SINGLE_DEPTH_NESTING);

But why is it safe?

Suppose we have two tasks, they both trace each other, both call
ptrace_stop() at the same time. Of course this is ugly, they both
will block.

But with this patch in this case we have the trivial ABBA deadlock,
no?

Oleg.