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BUG: KCSAN: data-race in fprop_new_period / fprop_reflect_period_percpu

write to 0xffffffff88f41780 of 4 bytes by interrupt on cpu 0:
 fprop_new_period+0x100/0x180 lib/flex_proportions.c:83
 writeout_period+0x55/0xe0 mm/page-writeback.c:628
 call_timer_fn+0x2e/0x240 kernel/time/timer.c:1431
 expire_timers+0x116/0x260 kernel/time/timer.c:1476
 __run_timers+0x358/0x3f0 kernel/time/timer.c:1745
 run_timer_softirq+0x2d/0x30 kernel/time/timer.c:1760
 __do_softirq+0x13c/0x2c3 kernel/softirq.c:345
 invoke_softirq kernel/softirq.c:221 [inline]
 __irq_exit_rcu+0xb4/0xc0 kernel/softirq.c:422
 sysvec_apic_timer_interrupt+0x37/0x80 arch/x86/kernel/apic/apic.c:1100
 asm_sysvec_apic_timer_interrupt+0x12/0x20 arch/x86/include/asm/idtentry.h:632

read to 0xffffffff88f41780 of 4 bytes by task 24 on cpu 1:
 fprop_reflect_period_percpu+0x22/0x1a0 lib/flex_proportions.c:192
 __fprop_inc_percpu lib/flex_proportions.c:222 [inline]
 __fprop_inc_percpu_max+0x3f/0x1f0 lib/flex_proportions.c:271
 wb_domain_writeout_inc mm/page-writeback.c:575 [inline]
 __wb_writeout_inc+0x7b/0x1c0 mm/page-writeback.c:599
 test_clear_page_writeback+0x3a4/0x530 mm/page-writeback.c:2745
 end_page_writeback+0xd7/0x260 mm/filemap.c:1460
 ext4_finish_bio+0x429/0x480 fs/ext4/page-io.c:143
 ext4_release_io_end+0x98/0x200 fs/ext4/page-io.c:158
 ext4_end_io_end fs/ext4/page-io.c:192 [inline]
 ext4_do_flush_completed_IO fs/ext4/page-io.c:257 [inline]
 ext4_end_io_rsv_work+0x306/0x360 fs/ext4/page-io.c:271
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2275
 worker_thread+0x616/0xa70 kernel/workqueue.c:2421
 kthread+0x20b/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 24 Comm: kworker/u4:1 Not tainted 5.12.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: ext4-rsv-conversion ext4_end_io_rsv_work
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