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Date:   Mon, 7 Oct 2019 09:34:53 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Marco Elver <elver@google.com>
Cc:     syzbot <syzbot+134336b86f728d6e55a0@syzkaller.appspotmail.com>,
        paulmck@kernel.org, josh@joshtriplett.org,
        mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com,
        Joel Fernandes <joel@joelfernandes.org>, rcu@vger.kernel.org,
        a@unstable.cc, b.a.t.m.a.n@lists.open-mesh.org,
        davem@davemloft.net, LKML <linux-kernel@vger.kernel.org>,
        mareklindner@neomailbox.ch, netdev@vger.kernel.org,
        sw@simonwunderlich.de, syzkaller-bugs@googlegroups.com
Subject: Re: KCSAN: data-race in find_next_bit / rcu_report_exp_cpu_mult
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On Mon, 7 Oct 2019 12:04:16 +0200
Marco Elver <elver@google.com> wrote:

> +RCU maintainers
> This might be a data-race in RCU itself.
> 
> >
> > write to 0xffffffff85a7f140 of 8 bytes by task 7 on cpu 0:
> >   rcu_report_exp_cpu_mult+0x4f/0xa0 kernel/rcu/tree_exp.h:244

Here we have:

	raw_spin_lock_irqsave_rcu_node(rnp, flags);
	if (!(rnp->expmask & mask)) {
		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
		return;
	}
	rnp->expmask &= ~mask;
	__rcu_report_exp_rnp(rnp, wake, flags); /* Releases rnp->lock. */

> >
> > read to 0xffffffff85a7f140 of 8 bytes by task 7251 on cpu 1:
> >   _find_next_bit lib/find_bit.c:39 [inline]
> >   find_next_bit+0x57/0xe0 lib/find_bit.c:70
> >   sync_rcu_exp_select_node_cpus+0x28e/0x510 kernel/rcu/tree_exp.h:375

and here we have:


	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

	/* IPI the remaining CPUs for expedited quiescent state. */
	for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) {


The write to rnp->expmask is done under the rnp->lock, but on the read
side, that lock is released before the for loop. Should we have
something like:

	unsigned long expmask;
	[...]

	expmask = rnp->expmask;
	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

	/* IPI the remaining CPUs for expedited quiescent state. */
	for_each_leaf_node_cpu_mask(rnp, cpu, expmask) {

?

-- Steve