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Date:   Mon, 7 Oct 2019 06:22:20 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Marco Elver <elver@google.com>
Cc:     syzbot <syzbot+134336b86f728d6e55a0@syzkaller.appspotmail.com>,
        josh@joshtriplett.org, rostedt@goodmis.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
Message-ID: <20191007132220.GZ2689@paulmck-ThinkPad-P72>
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 <CANpmjNNmSOagbTpffHr4=Yedckx9Rm2NuGqC9UqE+AOz5f1-ZQ@mail.gmail.com>
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On Mon, Oct 07, 2019 at 12:04:16PM +0200, Marco Elver wrote:
> +RCU maintainers
> This might be a data-race in RCU itself.

Quite possibly.  I will take a look, but there will be delays due to this
week being bootcamp and all.

							Thanx, Paul

> On Mon, 7 Oct 2019 at 12:01, syzbot
> <syzbot+134336b86f728d6e55a0@syzkaller.appspotmail.com> wrote:
> >
> > Hello,
> >
> > syzbot found the following crash on:
> >
> > HEAD commit:    b4bd9343 x86, kcsan: Enable KCSAN for x86
> > git tree:       https://github.com/google/ktsan.git kcsan
> > console output: https://syzkaller.appspot.com/x/log.txt?x=11edb20d600000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=c0906aa620713d80
> > dashboard link: https://syzkaller.appspot.com/bug?extid=134336b86f728d6e55a0
> > compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> >
> > Unfortunately, I don't have any reproducer for this crash yet.
> >
> > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > Reported-by: syzbot+134336b86f728d6e55a0@syzkaller.appspotmail.com
> >
> > ==================================================================
> > BUG: KCSAN: data-race in find_next_bit / rcu_report_exp_cpu_mult
> >
> > 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
> >   rcu_report_exp_rdp+0x6c/0x90 kernel/rcu/tree_exp.h:254
> >   rcu_preempt_deferred_qs_irqrestore+0x3bb/0x580 kernel/rcu/tree_plugin.h:475
> >   rcu_read_unlock_special+0xec/0x370 kernel/rcu/tree_plugin.h:659
> >   __rcu_read_unlock+0xcf/0xe0 kernel/rcu/tree_plugin.h:394
> >   rcu_read_unlock include/linux/rcupdate.h:645 [inline]
> >   batadv_nc_purge_orig_hash net/batman-adv/network-coding.c:411 [inline]
> >   batadv_nc_worker+0x13a/0x390 net/batman-adv/network-coding.c:718
> >   process_one_work+0x3d4/0x890 kernel/workqueue.c:2269
> >   worker_thread+0xa0/0x800 kernel/workqueue.c:2415
> >   kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
> >   ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352
> >
> > 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
> >   sync_rcu_exp_select_cpus+0x30c/0x590 kernel/rcu/tree_exp.h:439
> >   rcu_exp_sel_wait_wake kernel/rcu/tree_exp.h:575 [inline]
> >   wait_rcu_exp_gp+0x25/0x40 kernel/rcu/tree_exp.h:589
> >   process_one_work+0x3d4/0x890 kernel/workqueue.c:2269
> >   worker_thread+0xa0/0x800 kernel/workqueue.c:2415
> >   kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
> >   ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352
> >
> > Reported by Kernel Concurrency Sanitizer on:
> > CPU: 1 PID: 7251 Comm: kworker/1:4 Not tainted 5.3.0+ #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> > Google 01/01/2011
> > Workqueue: rcu_gp wait_rcu_exp_gp
> > ==================================================================
> >
> >
> > ---
> > This bug is generated by a bot. It may contain errors.
> > See https://goo.gl/tpsmEJ for more information about syzbot.
> > syzbot engineers can be reached at syzkaller@googlegroups.com.
> >
> > syzbot will keep track of this bug report. See:
> > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.