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BUG: KCSAN: data-race in __wb_update_bandwidth / balance_dirty_pages

write to 0xffff8880a945ed08 of 8 bytes by task 7882 on cpu 1:
 __wb_update_bandwidth+0x2cb/0x3d0 mm/page-writeback.c:1392
 wb_update_bandwidth+0x93/0xc0 mm/page-writeback.c:1399
 wb_writeback+0x21d/0x6a0 fs/fs-writeback.c:1897
 wb_check_old_data_flush fs/fs-writeback.c:1996 [inline]
 wb_do_writeback fs/fs-writeback.c:2049 [inline]
 wb_workfn+0x796/0x970 fs/fs-writeback.c:2078
 process_one_work+0x424/0x930 kernel/workqueue.c:2264
 worker_thread+0x9a/0x7e0 kernel/workqueue.c:2410
 kthread+0x1cb/0x1f0 kernel/kthread.c:255
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

read to 0xffff8880a945ed08 of 8 bytes by task 19125 on cpu 0:
 balance_dirty_pages+0x988/0x1ab0 mm/page-writeback.c:1706
 balance_dirty_pages_ratelimited+0x6f9/0xa20 mm/page-writeback.c:1913
 fault_dirty_shared_page+0x179/0x1a0 mm/memory.c:2421
 do_shared_fault mm/memory.c:3754 [inline]
 do_fault mm/memory.c:3806 [inline]
 handle_pte_fault mm/memory.c:4033 [inline]
 __handle_mm_fault+0x1df9/0x2cf0 mm/memory.c:4163
 handle_mm_fault+0x21c/0x540 mm/memory.c:4200
 do_user_addr_fault arch/x86/mm/fault.c:1442 [inline]
 do_page_fault+0x4a4/0xa52 arch/x86/mm/fault.c:1517
 page_fault+0x34/0x40 arch/x86/entry/entry_64.S:1203

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 19125 Comm: syz-executor.2 Not tainted 5.6.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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