linux-kernel - [PATCH 0/2] (Was: ptrace() hangs on attempt to seize/attach stopped & frozen task)

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Message-ID: <20151119184709.GA19061@redhat.com>
Date:	Thu, 19 Nov 2015 19:47:09 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:	Roland McGrath <roland@...k.frob.com>, Tejun Heo <tj@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Pedro Alves <palves@...hat.com>,
	Jan Kratochvil <jan.kratochvil@...hat.com>
Subject: [PATCH 0/2] (Was: ptrace() hangs on attempt to seize/attach
	stopped & frozen task)

On 11/09, Andrey Ryabinin wrote:
>
> So, the ptrace() hangs if we try to attach to stopped task from freezing cgroup.

OK, lets make it killable at least, TASK_KILLABLE looks better in
any case.

I am starting to think again that we should remove JOBCTL_TRAPPING,
but this needs a bit more discussion and probably testing.

Anyway I agree with Tejun, the freezer doesn't look nice too.

Hmm. I need to re-check but it seems we have another problem...
ptrace_stop() does task_participate_group_stop() and this (in particular)
clears JOBCTL_STOP_PENDING. This is only correct if the task is still traced,
but this is not necessarily true and we can in fact clear JOBCTL_STOP_PENDING
set by __ptrace_unlink() called by the exiting tracer.

Oleg.

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