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BUG: KCSAN: data-race in percpu_counter_add_batch / wb_over_bg_thresh

write to 0xffff8880a0e1a468 of 8 bytes by task 10982 on cpu 0:
 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]
 account_page_dirtied+0x1e8/0x490 mm/page-writeback.c:2427
 __set_page_dirty+0x86/0x270 fs/buffer.c:623
 mark_buffer_dirty+0x26e/0x2b0 fs/buffer.c:1152
 __block_commit_write.isra.0+0x11d/0x170 fs/buffer.c:2081
 block_write_end+0x6d/0x140 fs/buffer.c:2159
 generic_write_end+0x8c/0x1f0 fs/buffer.c:2173
 ext4_da_write_end+0x162/0x670 fs/ext4/inode.c:3092
 generic_perform_write+0x1d7/0x320 mm/filemap.c:3298
 ext4_buffered_write_iter+0x14e/0x280 fs/ext4/file.c:270
 ext4_file_write_iter+0xf4/0xd30 fs/ext4/file.c:642
 call_write_iter include/linux/fs.h:1901 [inline]
 new_sync_write+0x303/0x400 fs/read_write.c:483
 __vfs_write+0x9e/0xb0 fs/read_write.c:496
 vfs_write fs/read_write.c:558 [inline]
 vfs_write+0x189/0x380 fs/read_write.c:542
 ksys_write+0xc5/0x1a0 fs/read_write.c:611
 __do_sys_write fs/read_write.c:623 [inline]
 __se_sys_write fs/read_write.c:620 [inline]
 __x64_sys_write+0x49/0x60 fs/read_write.c:620
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880a0e1a468 of 8 bytes by task 7 on cpu 1:
 wb_stat include/linux/backing-dev.h:89 [inline]
 wb_over_bg_thresh+0x244/0x440 mm/page-writeback.c:1948
 wb_check_background_flush fs/fs-writeback.c:1952 [inline]
 wb_do_writeback fs/fs-writeback.c:2050 [inline]
 wb_workfn+0x38d/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: 7 Comm: kworker/u4:0 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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