Date:   Mon, 27 Feb 2023 08:32:58 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Zhouyi Zhou <zhouzhouyi@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Sanan Hasanov <sanan.hasanov@knights.ucf.edu>,
        frederic@kernel.org, quic_neeraju@quicinc.com,
        josh@joshtriplett.org, mathieu.desnoyers@efficios.com,
        jiangshanlai@gmail.com, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, syzkaller@googlegroups.com,
        contact@pgazz.com
Subject: Re: BUG: unable to handle kernel NULL pointer dereference in rcu_core
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Reply-To: paulmck@kernel.org
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On Mon, Feb 27, 2023 at 11:16:26AM -0500, Steven Rostedt wrote:
> On Tue, 28 Feb 2023 00:11:51 +0800
> Zhouyi Zhou <zhouzhouyi@gmail.com> wrote:
> 
> > > Yes certainly, the rcu_head is allocated on the caller side so it
> > > could have been trampled while the callback was still in flight.  
> > Thank you all for your guidance, I learned a lot during this process
> > >  
> > > > OR it could be a bug with RCU if the synchronize_rcu() ended before the
> > > > grace periods have finished.  
> > Thanks again.
> > 
> > By the way, the syzkaller on my local machine has been running for 8
> > hours, only three bugs reported[1][2][3], but they don't seem to be
> > related to Sanan's original report.
> > Maybe there are some configuration mismatches between us.The test
> > continues,  I will report to you once I have any new discovery.
> 
> Note, the above races (either bug, the one that tramples on something in
> RCU flight, or a synchronize_sched() returning early) may be extremely hard
> to hit. It could have been the planets were lined up just right to hit the
> bug, and won't happen for another 27,000 years.

Which turns into once per week or two across a million-system fleet.  ;-)

Not that I know of any fleets running syzkaller...

							Thanx, Paul