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BUG: KCSAN: data-race in __wb_update_bandwidth / percpu_counter_add_batch

write to 0xffff88812a7b6158 of 8 bytes by task 8040 on cpu 1:
 percpu_counter_add_batch+0xbc/0x140 lib/percpu_counter.c:91
 __add_wb_stat include/linux/backing-dev.h:74 [inline]
 inc_wb_stat include/linux/backing-dev.h:79 [inline]
 __wb_writeout_inc mm/page-writeback.c:604 [inline]
 test_clear_page_writeback+0x533/0x7e0 mm/page-writeback.c:2737
 end_page_writeback+0x98/0x180 mm/filemap.c:1317
 ext4_finish_bio+0x39b/0x460 fs/ext4/page-io.c:146
 ext4_release_io_end+0xc5/0x270 fs/ext4/page-io.c:161
 ext4_end_io_end fs/ext4/page-io.c:195 [inline]
 ext4_do_flush_completed_IO fs/ext4/page-io.c:260 [inline]
 ext4_end_io_rsv_work+0x159/0x400 fs/ext4/page-io.c:274
 process_one_work+0x424/0x930 kernel/workqueue.c:2268
 worker_thread+0x9a/0x7e0 kernel/workqueue.c:2414
 kthread+0x203/0x230 kernel/kthread.c:268
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

read to 0xffff88812a7b6158 of 8 bytes by task 28 on cpu 0:
 __wb_update_bandwidth+0xb0/0x3d0 mm/page-writeback.c:1365
 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:2268
 worker_thread+0x9a/0x7e0 kernel/workqueue.c:2414
 kthread+0x203/0x230 kernel/kthread.c:268
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 28 Comm: kworker/u4:2 Not tainted 5.7.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: writeback wb_workfn (flush-8:0)
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