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Date: Tue, 10 Nov 2015 21:20:07 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Roland McGrath <roland@hack.frob.com>, Tejun Heo <tj@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: ptrace() hangs on attempt to seize/attach stopped & frozen task
Message-ID: <20151110201950.GA4925@redhat.com>
References: <5640B7F2.70406@virtuozzo.com> <20151109185506.GA22744@redhat.com>
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On 11/09, Oleg Nesterov wrote:
>
> Hi,
>
> On 11/09, Andrey Ryabinin wrote:
> >
> > Hi,
> >
> > So, the ptrace() hangs if we try to attach to stopped task from freezing cgroup.
> > It seems this was introduced by 5d8f72b55c2756("freezer: change ptrace_stop/do_signal_stop to use freezable_schedule()").
>
> quite possible...

Not really, this was broken before that commit. Note that try_to_freeze()
was called under "relock:" label, so we had the same problem: the tracee
doesn't do do_jobctl_trap() after do_signal_stop() without try_to_freeze()
in between. Not that it matters, but still.

Oleg.