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Date:   Wed, 23 Nov 2022 14:17:30 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     Pengfei Xu <pengfei.xu@intel.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Christian Brauner <brauner@kernel.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        linux-kernel@vger.kernel.org, heng.su@intel.com,
        rcu@vger.kernel.org
Subject: Re: PID_NS unshare VS synchronize_rcu_tasks() (was: Re: [Syzkaller &
 bisect] There is task hung in "synchronize_rcu" in v6.1-rc5 kernel)
Message-ID: <20221123221730.GK4001@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
References: <Y3sOgrOmMQqPMItu@xpf.sh.intel.com>
 <20221123143758.GA1387380@lothringen>
 <Y35ALpl8borkSHjy@xpf.sh.intel.com>
 <20221123220648.GA1395324@lothringen>
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On Wed, Nov 23, 2022 at 11:06:48PM +0100, Frederic Weisbecker wrote:
> On Wed, Nov 23, 2022 at 11:45:50PM +0800, Pengfei Xu wrote:
> > On 2022-11-23 at 15:37:58 +0100, Frederic Weisbecker wrote:
> > > I have no idea how to solve the situation without violating the pid_namespace
> > > rules and unshare() semantics (although I wish unshare(CLONE_NEWPID) had a less
> > > error prone behaviour with allowing creating more than one task belonging to the
> > > same namespace).
> > > 
> > > So probably having an SRCU read side critical section within exit_notify() is
> > > not a good idea, is there a solution to work around that for rcu tasks?
> > > 
> >   Thanks for the analysis!
> >   Add one more information: I tried to revert this commit only on top of
> >   v6.1-rc5 mainline by script, but it caused kernel make to fail, it could not
> >   confirm the bisect information is 100% accurate if I could not pass the
> >   revert step verification. I just provide all the information I could.
> 
> No problem, I managed to reproduce with latest upstream.
> I don't think the bisected commit is the culprit though, it may perhaps just make
> the issue more likely to happen.

Frederic, Boqun, Neeraj, and I dug through this earlier today, and
record of our wanderings may be found here:

https://docs.google.com/document/d/1hJxgiZ5TMZ4YJkdJPLAkRvq7sYQ-A7svgA8no6i-v8k/edit?usp=sharing

It looks like we can break the deadlock within RCU Tasks, but it also
looks like the namespace-PID semantics are at best an accident waiting
to happen.  ;-)

							Thanx, Paul

> Thanks.
> 
> > 
> >   And this issue is too difficult to me.
> >   If I find more clue, I will update the eamil.
> > 
> >   Thanks!
> >   BR.
> > 
> > > Thanks.