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BUG: KCSAN: data-race in __wb_update_bandwidth / percpu_counter_add_batch

write to 0xffff8881299aa118 of 8 bytes by task 19124 on cpu 1:
 percpu_counter_add_batch+0xbd/0x130 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]
 account_page_dirtied+0x217/0x3d0 mm/page-writeback.c:2436
 __set_page_dirty+0xbb/0x1d0 fs/buffer.c:608
 mark_buffer_dirty+0x12e/0x260 fs/buffer.c:1137
 __block_commit_write fs/buffer.c:2084 [inline]
 block_commit_write fs/buffer.c:2476 [inline]
 block_page_mkwrite+0x25d/0x2e0 fs/buffer.c:2525
 ext4_page_mkwrite+0x572/0xae0 fs/ext4/inode.c:5985
 do_page_mkwrite mm/memory.c:2730 [inline]
 do_shared_fault+0x21e/0x7c0 mm/memory.c:4096
 do_fault mm/memory.c:4164 [inline]
 handle_pte_fault mm/memory.c:4400 [inline]
 __handle_mm_fault mm/memory.c:4535 [inline]
 handle_mm_fault+0x1710/0x1af0 mm/memory.c:4633
 do_user_addr_fault+0x380/0x770 arch/x86/mm/fault.c:1372
 handle_page_fault arch/x86/mm/fault.c:1429 [inline]
 exc_page_fault+0xb8/0x330 arch/x86/mm/fault.c:1482
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:538

read to 0xffff8881299aa118 of 8 bytes by task 31162 on cpu 0:
 percpu_counter_read include/linux/percpu_counter.h:72 [inline]
 __wb_update_bandwidth+0x8d/0x5d0 mm/page-writeback.c:1358
 wb_update_bandwidth+0x59/0x80 mm/page-writeback.c:1393
 wb_writeback+0x2c3/0x6f0 fs/fs-writeback.c:1899
 wb_do_writeback+0x101/0x5f0 fs/fs-writeback.c:2039
 wb_workfn+0xc8/0x470 fs/fs-writeback.c:2080
 process_one_work+0x3e1/0x9a0 kernel/workqueue.c:2269
 worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
 kthread+0x20d/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 31162 Comm: kworker/u4:1 Not tainted 5.9.0-rc7-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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