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

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Message-ID: <4df1e23a-8b07-8439-c54b-6ef9864aa78a@kernel.org>
Date:   Mon, 20 Jul 2020 13:40:20 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     peterz@...radead.org, Oleg Nesterov <oleg@...hat.com>
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>,
        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 20. 07. 20, 13:26, peterz@...radead.org wrote:
> On Mon, Jul 20, 2020 at 12:59:24PM +0200, peterz@...radead.org wrote:
>> On Mon, Jul 20, 2020 at 10:41:06AM +0200, Peter Zijlstra wrote:
>>> 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.
> 
> This then? That seems to survive the strace thing.

FWIW for me too.

thanks,
-- 
js