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BUG: KCSAN: data-race in balance_dirty_pages / percpu_counter_add_batch

write to 0xffff88809bb42090 of 8 bytes by task 23913 on cpu 1:
 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]
 test_clear_page_writeback+0x4fe/0x7e0 mm/page-writeback.c:2736
 end_page_writeback+0x98/0x180 mm/filemap.c:1317
 ext4_finish_bio+0x430/0x540 fs/ext4/page-io.c:148
 ext4_release_io_end+0xc5/0x270 fs/ext4/page-io.c:163
 ext4_end_io_end fs/ext4/page-io.c:197 [inline]
 ext4_do_flush_completed_IO fs/ext4/page-io.c:262 [inline]
 ext4_end_io_rsv_work+0x159/0x400 fs/ext4/page-io.c:276
 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

read to 0xffff88809bb42090 of 8 bytes by task 31448 on cpu 0:
 wb_stat include/linux/backing-dev.h:89 [inline]
 wb_dirty_limits mm/page-writeback.c:1550 [inline]
 balance_dirty_pages+0x11e1/0x1ab0 mm/page-writeback.c:1693
 balance_dirty_pages_ratelimited+0x6f9/0xa20 mm/page-writeback.c:1913
 generic_perform_write+0x224/0x320 mm/filemap.c:3323
 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

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 31448 Comm: syz-executor.4 Not tainted 5.6.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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