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

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Message-ID: <20250919161353.GB22933@redhat.com>
Date: Fri, 19 Sep 2025 18:13:54 +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)

Let me repeat that currently I have no idea, so let me ask another stupid
question...

On 09/19, Matt Fleming wrote:
>
> I do have some info. The callstack for the lost thread is:
>
> Call Trace:
>  <TASK>
>  __schedule+0x4fb/0xbf0
>  ? srso_return_thunk+0x5/0x5f
>  schedule+0x27/0xf0
>  do_exit+0xdd/0xaa0
>  ? __pfx_futex_wake_mark+0x10/0x10
>  do_group_exit+0x30/0x80
>  get_signal+0x81e/0x860
>  ? srso_return_thunk+0x5/0x5f
>  ? futex_wake+0x177/0x1a0
>  arch_do_signal_or_restart+0x2e/0x1f0
>  ? srso_return_thunk+0x5/0x5f
>  ? srso_return_thunk+0x5/0x5f
>  ? __x64_sys_futex+0x10c/0x1d0
>  syscall_exit_to_user_mode+0xa5/0x130
>  do_syscall_64+0x57/0x110
>  entry_SYSCALL_64_after_hwframe+0x76/0x7e

OK, thanks. Nothing "interesting" at first glance.

> do_exit+0xdd is here in coredump_task_wait():
>
>                 for (;;) {
>                         set_current_state(TASK_IDLE|TASK_FREEZABLE);
>                         if (!self.task) /* see coredump_finish() */
>                                 break;
>                         schedule();
>                 }
>
> i.e. the task calls schedule() and never comes back.

Are you sure it never comes back and doesn't loop?

> The waiting task
> sees p->on_rq=1 for this lost thread

Strange...

Oleg.