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BUG: KCSAN: data-race in __wb_update_bandwidth / percpu_counter_add_batch

read-write to 0xffff8881014d0540 of 8 bytes by task 8 on cpu 1:
 percpu_counter_add_batch+0x9b/0x100 lib/percpu_counter.c:91
 __add_wb_stat+0x4c/0x60 include/linux/backing-dev.h:70
 inc_wb_stat include/linux/backing-dev.h:75 [inline]
 __wb_writeout_inc+0x27/0x1a0 mm/page-writeback.c:598
 test_clear_page_writeback+0x2fa/0x450 mm/page-writeback.c:2745
 end_page_writeback+0xa7/0x110 mm/filemap.c:1487
 ext4_finish_bio+0x429/0x490 fs/ext4/page-io.c:146
 ext4_release_io_end+0x98/0x200 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+0x306/0x360 fs/ext4/page-io.c:274
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2272
 worker_thread+0x635/0xb90 kernel/workqueue.c:2418
 kthread+0x1fa/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

read to 0xffff8881014d0540 of 8 bytes by task 26 on cpu 0:
 percpu_counter_read include/linux/percpu_counter.h:72 [inline]
 __wb_update_bandwidth+0xa7/0x5c0 mm/page-writeback.c:1359
 wb_update_bandwidth+0x59/0x80 mm/page-writeback.c:1393
 wb_writeback+0x2a3/0x660 fs/fs-writeback.c:1899
 wb_check_old_data_flush fs/fs-writeback.c:1998 [inline]
 wb_do_writeback+0x520/0x5d0 fs/fs-writeback.c:2051
 wb_workfn+0xb8/0x410 fs/fs-writeback.c:2080
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2272
 worker_thread+0x635/0xb90 kernel/workqueue.c:2418
 kthread+0x1fa/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 26 Comm: kworker/u4:2 Not tainted 5.10.0-rc3-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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