linux-kernel - Re: [RCU] zombie task hung in synchronize_rcu

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Message-ID: <20240607212116.GA7258@redhat.com>
Date: Fri, 7 Jun 2024 23:22:59 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Wei Fu <fuweid89@...il.com>
Cc: Sudhanva.Huruli@...rosoft.com, akpm@...ux-foundation.org,
	apais@...ux.microsoft.com, axboe@...nel.dk, boqun.feng@...il.com,
	brauner@...nel.org, ebiederm@...ssion.com, frederic@...nel.org,
	j.granados@...sung.com, jiangshanlai@...il.com,
	joel@...lfernandes.org, josh@...htriplett.org,
	linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
	michael.christie@...cle.com, mjguzik@...il.com,
	neeraj.upadhyay@...nel.org, paulmck@...nel.org,
	qiang.zhang1211@...il.com, rachelmenge@...ux.microsoft.com,
	rcu@...r.kernel.org, rostedt@...dmis.org, weifu@...rosoft.com
Subject: Re: [RCU] zombie task hung in synchronize_rcu_expedited

On 06/07, Wei Fu wrote:
>
> Yes. I applied your patch on v5.15.160 and run reproducer for 5 hours.
> I didn't see this issue. Currently, it looks good!. I will continue that test
> on this weekend.

Great, thanks!

> In last reply, you mentioned TIF_NOTIFY_SIGNAL related to busy-wait loop.
> Would you please explain why flag-clear works here?

Sure, I'll write the changelog with the explanation and send the patch on
weekend. If it passes your testing.

But in short this is very simple. zap_pid_ns_processes() clears TIF_SIGPENDING
exactly because we want to avoid the busy-wait loop. But today this is not
enough to make signal_pending() return F, see
include/linux/sched/signal.h:signal_pending().

Thanks,

Oleg.