linux-kernel - Re: 5.8-rc*: kernel BUG at kernel/signal.c:1917

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Message-ID: <20200720064326.GA6612@redhat.com>
Date:   Mon, 20 Jul 2020 08:43:26 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Jiri Slaby <jirislaby@...nel.org>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        christian@...uner.io, "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linux kernel mailing list <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Dave Jones <davej@...emonkey.org.uk>,
        Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: Re: 5.8-rc*: kernel BUG at kernel/signal.c:1917

On 07/20, Jiri Slaby wrote:
>
> You tackled it, we cherry-picked dbfb089d360 to our kernels. Ccing more
> people.

Thanks... so with this patch __schedule() does

	prev_state = prev->state;

	...

	if (!preempt && prev_state && prev_state == prev->state) {
		if (signal_pending_state(prev_state, prev)) {
			prev->state = TASK_RUNNING;
		} else {

and ptrace_freeze_traced() can change ->state in between. This means
that this task can return from __schedule() with ->state != RUNNING,
this can explain BUG_ON(task_is_stopped_or_traced) in do_notify_parent()
you reported.

Oleg.