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

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Message-ID: <20200720084106.GJ10769@hirez.programming.kicks-ass.net>
Date:   Mon, 20 Jul 2020 10:41:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Jiri Slaby <jirislaby@...nel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        christian@...uner.io, "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linux kernel mailing list <linux-kernel@...r.kernel.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 Mon, Jul 20, 2020 at 10:26:58AM +0200, Oleg Nesterov wrote:
> Peter,
> 
> Let me add another note. TASK_TRACED/TASK_STOPPED was always protected by
> ->siglock. In particular, ttwu(__TASK_TRACED) must be always called with
> ->siglock held. That is why ptrace_freeze_traced() assumes it can safely
> do s/TASK_TRACED/__TASK_TRACED/ under spin_lock(siglock).
> 
> Can this change race with
> 
> 		if (signal_pending_state(prev->state, prev)) {
> 			prev->state = TASK_RUNNING;
> 		}
> 
> in __schedule() ? Hopefully not, signal-state is protected by siglock too.
> 
> So I think this logic was correct even if it doesn't look nice. But "doesn't
> look nice" is true for the whole ptrace code ;)

*groan*... another bit of obscure magic :-(

let me go try and wake up and figure out how best to deal with this.