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BUG: KCSAN: data-race in percpu_counter_add_batch / wb_over_bg_thresh

read to 0xffff88812a3be0e0 of 8 bytes by task 7604 on cpu 1:
 wb_stat include/linux/backing-dev.h:88 [inline]
 wb_over_bg_thresh+0x247/0x490 mm/page-writeback.c:1948
 wb_check_background_flush fs/fs-writeback.c:1952 [inline]
 wb_do_writeback fs/fs-writeback.c:2050 [inline]
 wb_workfn+0x3b3/0x970 fs/fs-writeback.c:2078
 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

write to 0xffff88812a3be0e0 of 8 bytes by task 7582 on cpu 0:
 percpu_counter_add_batch+0xca/0x150 lib/percpu_counter.c:91
 __add_wb_stat include/linux/backing-dev.h:73 [inline]
 inc_wb_stat include/linux/backing-dev.h:78 [inline]
 account_page_dirtied+0x1f7/0x4a0 mm/page-writeback.c:2427
 __set_page_dirty+0x8a/0x270 fs/buffer.c:583
 __set_page_dirty_buffers+0x1cf/0x280 fs/buffer.c:643
 ext4_set_page_dirty+0xc0/0x190 fs/ext4/inode.c:3899
 set_page_dirty+0x126/0x2b0 mm/page-writeback.c:2572
 fault_dirty_shared_page.isra.0+0x49/0x100 mm/memory.c:2237
 wp_page_shared mm/memory.c:2523 [inline]
 do_wp_page+0xdf9/0x11f0 mm/memory.c:2619
 handle_pte_fault mm/memory.c:3865 [inline]
 __handle_mm_fault+0x1ab1/0x2c70 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+0x456/0x8d0 arch/x86/mm/fault.c:1506
 do_page_fault+0x38/0x194 arch/x86/mm/fault.c:1530
 page_fault+0x34/0x40 arch/x86/entry/entry_64.S:1202

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 7582 Comm: syz-fuzzer Not tainted 5.4.0-rc7+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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