Re: KCSAN: data-race in find_next_bit / rcu_report_exp_cpu_mult

[Marco Elver]

+RCU maintainers
This might be a data-race in RCU itself.

On Mon, 7 Oct 2019 at 12:01, syzbot
<syzbot+134336b86f728d6e55a0@xxxxxxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> ==================================================================
> 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@xxxxxxxxxxxxxxxxx
>
> syzbot will keep track of this bug report. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.