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BUG: KCSAN: data-race in fprop_reflect_period_percpu.isra.0 / fprop_reflect_period_percpu.isra.0

write to 0xffff888129d609e0 of 4 bytes by task 25 on cpu 0:
 fprop_reflect_period_percpu.isra.0+0x127/0x1c0 lib/flex_proportions.c:215
 fprop_fraction_percpu+0x86/0x1f0 lib/flex_proportions.c:236
 __wb_calc_thresh+0x6e/0x240 mm/page-writeback.c:775
 wb_calc_thresh+0x9b/0xd0 mm/page-writeback.c:795
 wb_over_bg_thresh+0x28f/0x460 mm/page-writeback.c:1949
 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:2268
 worker_thread+0x9a/0x7e0 kernel/workqueue.c:2414
 kthread+0x203/0x230 kernel/kthread.c:268
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

read to 0xffff888129d609e0 of 4 bytes by interrupt on cpu 1:
 fprop_reflect_period_percpu.isra.0+0x2e/0x1c0 lib/flex_proportions.c:196
 __fprop_inc_percpu_max+0x63/0x140 lib/flex_proportions.c:270
 wb_domain_writeout_inc mm/page-writeback.c:581 [inline]
 __wb_writeout_inc mm/page-writeback.c:605 [inline]
 test_clear_page_writeback+0x573/0x7e0 mm/page-writeback.c:2737
 end_page_writeback+0x98/0x180 mm/filemap.c:1317
 end_buffer_async_write+0x22d/0x260 fs/buffer.c:384
 end_bio_bh_io_sync+0x7d/0xa0 fs/buffer.c:3012
 bio_endio+0x2db/0x3e0 block/bio.c:1422
 req_bio_endio block/blk-core.c:245 [inline]
 blk_update_request+0x418/0x7a0 block/blk-core.c:1472
 blk_mq_end_request+0x4e/0x2f0 block/blk-mq.c:546
 lo_complete_rq+0x141/0x1a0 drivers/block/loop.c:490
 blk_done_softirq+0x1e6/0x250 block/blk-softirq.c:37
 __do_softirq+0x118/0x34a kernel/softirq.c:292
 run_ksoftirqd+0x41/0x60 kernel/softirq.c:604
 smpboot_thread_fn+0x374/0x4a0 kernel/smpboot.c:165
 kthread+0x203/0x230 kernel/kthread.c:268
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 16 Comm: ksoftirqd/1 Not tainted 5.7.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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