Subject: KCSAN: data-race in find_next_bit / rcu_report_exp_cpu_mult

From: syzbot <[email-masked]>
Date: Mon, 07 Oct 2019 03:01:06 -0700

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: [email-masked]

==================================================================
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
==================================================================

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