linux-kernel - Re: [PATCH 1/1] signal: on exit skip waiting for an ack from the tracer if it is frozen

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Message-ID: <20240703164836.GC28444@redhat.com>
Date: Wed, 3 Jul 2024 18:48:36 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: akpm@...ux-foundation.org, mhocko@...e.com, brauner@...nel.org,
	tandersen@...flix.com, bigeasy@...utronix.de,
	vincent.whitchurch@...s.com, ardb@...nel.org,
	linux-kernel@...r.kernel.org, Martin Liu <liumartin@...gle.com>,
	Minchan Kim <minchan@...gle.com>
Subject: Re: [PATCH 1/1] signal: on exit skip waiting for an ack from the
 tracer if it is frozen

Suren, I am sorry for the late reply,

On 06/30, Suren Baghdasaryan wrote:
>
> > I think it would better to simply change ptrace_stop() to check TIF_MEMDIE
> > along with __fatal_signal_pending() and return in this case.
>
> I think this would not fix the case we are experiencing. In our case
> the tracee is killed from the userspace (TIF_MEMDIE is not set yet),

OK, I misunderstood the problem.

> gets stuck in ptrace_stop() waiting for an ack from the tracer and
> then is picked up by OOM-killer with the abovementioned consequences.

and __task_will_free_mem() returns true if SIGNAL_GROUP_EXIT is set...
Nevermind.

> > Of course, this won't fix all problems.
>
> As I mentioned, I'm not an expert in ptrace, so I'll gladly try any
> better solution if one is proposed.

I do not see any solution, sorry.

ptrace doesn't allow to intercept/nack SIGKILL, but at the same time it
happily allows the killed tracee to sleep in PTRACE_EVENT_EXIT. And even
another SIGKILL/whatever can't wake the tracee up.

This is historical behaviour, I do not see how can we change it. Any
change will break something.

Oleg.