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BUG: KCSAN: data-race in __wb_calc_thresh / __wb_update_bandwidth

write to 0xffff8880a9592128 of 8 bytes by task 5272 on cpu 0:
 wb_update_write_bandwidth mm/page-writeback.c:1130 [inline]
 __wb_update_bandwidth+0x277/0x3c0 mm/page-writeback.c:1387
 wb_update_bandwidth+0x90/0xc0 mm/page-writeback.c:1399
 wb_writeback+0x21d/0x6a0 fs/fs-writeback.c:1894
 wb_check_background_flush fs/fs-writeback.c:1959 [inline]
 wb_do_writeback fs/fs-writeback.c:2047 [inline]
 wb_workfn+0x696/0x970 fs/fs-writeback.c:2075
 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 0xffff8880a9592128 of 8 bytes by task 21084 on cpu 1:
 __wb_calc_thresh+0xdc/0x250 mm/page-writeback.c:782
 wb_dirty_limits mm/page-writeback.c:1531 [inline]
 balance_dirty_pages+0x1187/0x1ba0 mm/page-writeback.c:1693
 balance_dirty_pages_ratelimited+0x752/0xb40 mm/page-writeback.c:1913
 fault_dirty_shared_page.isra.0+0x6e/0x100 mm/memory.c:2253
 do_shared_fault mm/memory.c:3570 [inline]
 do_fault mm/memory.c:3622 [inline]
 handle_pte_fault mm/memory.c:3849 [inline]
 __handle_mm_fault+0x212b/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+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

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 21084 Comm: syz-executor.1 Not tainted 5.4.0-rc6+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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