linux-kernel - Re: Debugging lost task in wait_task_inactive() when delivering signal (6.12)

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Message-ID: <20250919143611.GA22933@redhat.com>
Date: Fri, 19 Sep 2025 16:37:26 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Matt Fleming <mfleming@...udflare.com>
Cc: Peter Zijlstra <peterz@...radead.org>, John Stultz <jstultz@...gle.com>,
	kernel-team <kernel-team@...udflare.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Chris Arges <carges@...udflare.com>
Subject: Re: Debugging lost task in wait_task_inactive() when delivering
 signal (6.12)

Hi Matt,

On 09/19, Matt Fleming wrote:
>
> Hi there,
>
> We're running into an intermittent issue where tasks end up in a state with
> p->on_rq=1 and p->se.on_rq=0 when delivering a fatal signal to a thread
> group. The thread handling the signal sits in wait_task_inactive() after
> sending a SIGKILL to all other threads, most of which pass through
> coredump_task_exit() just fine, but occasionally one thread calls into
> coredump_task_exit()->schedule() and never comes back because of the above
> state.

I guess you mean coredump_wait() -> wait_task_inactive() ... Sorry I have no
clue at least right now... And I don't see any problem in coredump_task_exit().

Stupid question. Any chance you can reproduce, figure out the pid of that
sub-thread which fools wait_task_inactive() and, say, do
"cat /proc/pid-of-that-thread/stack" ? Or any other info, everything can
help. Crash dump? Yes, you have already mentioned this is hard-to-reproduce :(

Oleg.