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BUG: KCSAN: data-race in fprop_fraction_percpu / percpu_counter_add_batch

read to 0xffffffff86047770 of 8 bytes by task 8253 on cpu 0:
 fprop_fraction_percpu+0x9c/0x1c0 lib/flex_proportions.c:238
 __wb_calc_thresh+0x6f/0x250 mm/page-writeback.c:775
 wb_calc_thresh+0x96/0xc0 mm/page-writeback.c:795
 wb_over_bg_thresh+0x287/0x490 mm/page-writeback.c:1949
 wb_writeback+0x4bc/0x6a0 fs/fs-writeback.c:1870
 wb_check_background_flush fs/fs-writeback.c:1959 [inline]
 wb_do_writeback fs/fs-writeback.c:2047 [inline]
 wb_workfn+0x696/0x970 fs/fs-writeback.c:2075
 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 0xffffffff86047770 of 8 bytes by task 7 on cpu 1:
 percpu_counter_add_batch+0xca/0x150 lib/percpu_counter.c:91
 percpu_counter_add include/linux/percpu_counter.h:55 [inline]
 __fprop_inc_percpu_max+0xb9/0x150 lib/flex_proportions.c:272
 wb_domain_writeout_inc mm/page-writeback.c:581 [inline]
 __wb_writeout_inc mm/page-writeback.c:605 [inline]
 test_clear_page_writeback+0x540/0x790 mm/page-writeback.c:2737
 end_page_writeback+0x9e/0x190 mm/filemap.c:1339
 __block_write_full_page+0x58d/0x870 fs/buffer.c:1783
 block_write_full_page+0x1c0/0x1e0 fs/buffer.c:2953
 blkdev_writepage+0x2e/0x40 fs/block_dev.c:609
 __writepage+0x40/0xb0 mm/page-writeback.c:2303
 write_cache_pages+0x47a/0xb40 mm/page-writeback.c:2238
 generic_writepages mm/page-writeback.c:2329 [inline]
 generic_writepages+0x79/0xc0 mm/page-writeback.c:2318
 blkdev_writepages+0x27/0x40 fs/block_dev.c:2031
 do_writepages+0x6b/0x170 mm/page-writeback.c:2344
 __writeback_single_inode+0xaf/0x8e0 fs/fs-writeback.c:1449
 writeback_sb_inodes+0x4ba/0xa50 fs/fs-writeback.c:1713
 __writeback_inodes_wb+0xe8/0x1b0 fs/fs-writeback.c:1782
 wb_writeback+0x557/0x6a0 fs/fs-writeback.c:1891
 wb_check_start_all fs/fs-writeback.c:2015 [inline]
 wb_do_writeback fs/fs-writeback.c:2041 [inline]
 wb_workfn+0x748/0x970 fs/fs-writeback.c:2075
 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

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 7 Comm: kworker/u4:0 Not tainted 5.4.0-rc6+ #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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