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Date:   Mon, 04 Nov 2019 03:30:07 -0800
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Subject: KCSAN: data-race in __rcu_read_unlock / sync_rcu_exp_select_cpus
From:   syzbot <syzbot+99f4ddade3c22ab0cf23@syzkaller.appspotmail.com>
To:     akpm@linux-foundation.org, amir73il@gmail.com,
        darrick.wong@oracle.com, elver@google.com, hannes@cmpxchg.org,
        jack@suse.cz, josef@toxicpanda.com, linux-kernel@vger.kernel.org,
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Hello,

syzbot found the following crash on:

HEAD commit:    05f22368 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=1296ff50e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=87d111955f40591f
dashboard link: https://syzkaller.appspot.com/bug?extid=99f4ddade3c22ab0cf23
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+99f4ddade3c22ab0cf23@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in __rcu_read_unlock / sync_rcu_exp_select_cpus

read to 0xffffffff85a7d440 of 8 bytes by task 7624 on cpu 0:
  rcu_read_unlock_special kernel/rcu/tree_plugin.h:615 [inline]
  __rcu_read_unlock+0x381/0x3c0 kernel/rcu/tree_plugin.h:383
  rcu_read_unlock include/linux/rcupdate.h:652 [inline]
  filemap_map_pages+0x5b3/0x990 mm/filemap.c:2687
  do_fault_around mm/memory.c:3450 [inline]
  do_read_fault mm/memory.c:3484 [inline]
  do_fault mm/memory.c:3618 [inline]
  handle_pte_fault mm/memory.c:3849 [inline]
  __handle_mm_fault+0x2554/0x2cb0 mm/memory.c:3973
  handle_mm_fault+0x21b/0x530 mm/memory.c:4010
  do_user_addr_fault arch/x86/mm/fault.c:1441 [inline]
  __do_page_fault+0x3fb/0x9e0 arch/x86/mm/fault.c:1506
  do_page_fault+0x54/0x233 arch/x86/mm/fault.c:1530
  page_fault+0x34/0x40 arch/x86/entry/entry_64.S:1202

write to 0xffffffff85a7d440 of 8 bytes by task 7353 on cpu 1:
  sync_exp_reset_tree kernel/rcu/tree_exp.h:137 [inline]
  sync_rcu_exp_select_cpus+0xd5/0x590 kernel/rcu/tree_exp.h:427
  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: 7353 Comm: kworker/1:0 Not tainted 5.4.0-rc3+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Workqueue: rcu_gp wait_rcu_exp_gp
==================================================================
Kernel panic - not syncing: panic_on_warn set ...
CPU: 1 PID: 7353 Comm: kworker/1:0 Not tainted 5.4.0-rc3+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Workqueue: rcu_gp wait_rcu_exp_gp
Call Trace:
  __dump_stack lib/dump_stack.c:77 [inline]
  dump_stack+0xf5/0x159 lib/dump_stack.c:113
  panic+0x210/0x640 kernel/panic.c:221
  kcsan_report.cold+0xc/0x10 kernel/kcsan/report.c:302
  __kcsan_setup_watchpoint+0x32e/0x4a0 kernel/kcsan/core.c:411
  __tsan_write8 kernel/kcsan/kcsan.c:36 [inline]
  __tsan_write8+0x32/0x40 kernel/kcsan/kcsan.c:36
  sync_exp_reset_tree kernel/rcu/tree_exp.h:137 [inline]
  sync_rcu_exp_select_cpus+0xd5/0x590 kernel/rcu/tree_exp.h:427
  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
Shutting down cpus with NMI
Kernel Offset: disabled
Rebooting in 86400 seconds..


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