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BUG: KCSAN: data-race in percpu_counter_add_batch / wb_over_bg_thresh

read to 0xffff888121449068 of 8 bytes by task 2646 on cpu 0:
 wb_stat include/linux/backing-dev.h:88 [inline]
 wb_over_bg_thresh+0x247/0x490 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+0x3b3/0x970 fs/fs-writeback.c:2078
 process_one_work+0x3d4/0x890 kernel/workqueue.c:2269
 worker_thread+0xa0/0x800 kernel/workqueue.c:2415
 kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

write to 0xffff888121449068 of 8 bytes by task 6942 on cpu 1:
 percpu_counter_add_batch+0xca/0x150 lib/percpu_counter.c:91
 __add_wb_stat include/linux/backing-dev.h:73 [inline]
 inc_wb_stat include/linux/backing-dev.h:78 [inline]
 account_page_dirtied+0x1f7/0x4a0 mm/page-writeback.c:2427
 __set_page_dirty+0x8a/0x270 fs/buffer.c:583
 mark_buffer_dirty+0x27f/0x2c0 fs/buffer.c:1112
 __block_commit_write.isra.0+0x11f/0x170 fs/buffer.c:2041
 block_write_end+0x6d/0x140 fs/buffer.c:2119
 generic_write_end+0x8a/0x1f0 fs/buffer.c:2133
 ext4_da_write_end+0x158/0x620 fs/ext4/inode.c:3189
 generic_perform_write+0x1d3/0x320 mm/filemap.c:3316
 __generic_file_write_iter+0x251/0x380 mm/filemap.c:3434
 ext4_file_write_iter+0x1bd/0xa00 fs/ext4/file.c:268
 call_write_iter include/linux/fs.h:1895 [inline]
 new_sync_write+0x388/0x4a0 fs/read_write.c:483
 __vfs_write+0xb1/0xc0 fs/read_write.c:496
 vfs_write fs/read_write.c:558 [inline]
 vfs_write+0x18a/0x390 fs/read_write.c:542
 ksys_write+0xd5/0x1b0 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+0x4c/0x60 fs/read_write.c:620
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 6942 Comm: syz-executor.2 Not tainted 5.4.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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