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BUG: KCSAN: data-race in percpu_counter_add_batch / wb_over_bg_thresh

write to 0xffff8880aa2d3868 of 8 bytes by task 13259 on cpu 0:
 percpu_counter_add_batch+0xbc/0x140 lib/percpu_counter.c:91
 __add_wb_stat include/linux/backing-dev.h:74 [inline]
 dec_wb_stat include/linux/backing-dev.h:84 [inline]
 account_page_cleaned mm/page-writeback.c:2448 [inline]
 account_page_cleaned+0x221/0x2f0 mm/page-writeback.c:2442
 __cancel_dirty_page+0x257/0x300 mm/page-writeback.c:2629
 cancel_dirty_page include/linux/mm.h:1592 [inline]
 try_to_free_buffers+0x25f/0x2a0 fs/buffer.c:3307
 ext4_releasepage+0x1cf/0x1e0 fs/ext4/inode.c:3296
 try_to_release_page+0x149/0x1c0 mm/filemap.c:3483
 block_invalidatepage+0x30a/0x320 fs/buffer.c:1541
 ext4_invalidatepage+0x76/0x1d0 fs/ext4/inode.c:3256
 do_invalidatepage mm/truncate.c:163 [inline]
 truncate_cleanup_page+0xb8/0x230 mm/truncate.c:185
 truncate_inode_pages_range+0x4f4/0x1010 mm/truncate.c:365
 truncate_inode_pages mm/truncate.c:476 [inline]
 truncate_pagecache+0x5b/0x80 mm/truncate.c:817
 ext4_setattr+0xc32/0x1290 fs/ext4/inode.c:5357
 notify_change+0x82b/0xb00 fs/attr.c:336
 do_truncate+0xf9/0x180 fs/open.c:64
 handle_truncate fs/namei.c:3083 [inline]
 do_last fs/namei.c:3496 [inline]
 path_openat+0x1241/0x2f70 fs/namei.c:3607
 do_filp_open+0x11e/0x1b0 fs/namei.c:3637
 do_sys_openat2+0x4f5/0x620 fs/open.c:1149
 do_sys_open+0xa2/0x110 fs/open.c:1165
 ksys_open include/linux/syscalls.h:1386 [inline]
 __do_sys_creat fs/open.c:1233 [inline]
 __se_sys_creat fs/open.c:1231 [inline]
 __x64_sys_creat+0x42/0x60 fs/open.c:1231
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880aa2d3868 of 8 bytes by task 23544 on cpu 1:
 wb_stat include/linux/backing-dev.h:89 [inline]
 wb_over_bg_thresh+0x244/0x440 mm/page-writeback.c:1948
 wb_writeback+0x4bc/0x6a0 fs/fs-writeback.c:1873
 wb_check_background_flush fs/fs-writeback.c:1962 [inline]
 wb_do_writeback fs/fs-writeback.c:2050 [inline]
 wb_workfn+0x66b/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

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 23544 Comm: kworker/u4:1 Not tainted 5.6.0-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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