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BUG: KCSAN: data-race in fprop_reflect_period_percpu / fprop_reflect_period_percpu

write to 0xffff8881020c79c8 of 4 bytes by task 30 on cpu 1:
 fprop_reflect_period_percpu+0x161/0x190 lib/flex_proportions.c:135
 fprop_fraction_percpu+0x77/0x150 lib/flex_proportions.c:157
 __wb_calc_thresh+0x89/0x2d0 mm/page-writeback.c:905
 wb_bg_dirty_limits mm/page-writeback.c:2125 [inline]
 domain_over_bg_thresh mm/page-writeback.c:2139 [inline]
 wb_over_bg_thresh+0x222/0x530 mm/page-writeback.c:2160
 wb_check_background_flush fs/fs-writeback.c:2303 [inline]
 wb_do_writeback fs/fs-writeback.c:2401 [inline]
 wb_workfn+0x664/0x930 fs/fs-writeback.c:2428
 process_one_work kernel/workqueue.c:3314 [inline]
 process_scheduled_works+0x4f0/0x9c0 kernel/workqueue.c:3397
 worker_thread+0x58a/0x780 kernel/workqueue.c:3478
 kthread+0x22a/0x280 kernel/kthread.c:436
 ret_from_fork+0x146/0x330 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

read to 0xffff8881020c79c8 of 4 bytes by interrupt on cpu 0:
 fprop_reflect_period_percpu+0x31/0x190 lib/flex_proportions.c:116
 __fprop_add_percpu lib/flex_proportions.c:143 [inline]
 __fprop_add_percpu_max+0x57/0x190 lib/flex_proportions.c:201
 wb_domain_writeout_add mm/page-writeback.c:554 [inline]
 __wb_writeout_add+0x75/0x1b0 mm/page-writeback.c:578
 __folio_end_writeback+0x14b/0x360 mm/page-writeback.c:2970
 folio_end_writeback_no_dropbehind+0x6d/0x1d0 mm/filemap.c:1667
 folio_end_writeback+0x1c/0x70 mm/filemap.c:1693
 ext4_finish_bio+0x450/0x760 fs/ext4/page-io.c:144
 ext4_end_bio+0x27b/0x380 fs/ext4/page-io.c:392
 bio_endio+0x35b/0x3f0 block/bio.c:1798
 blk_update_request+0x338/0x710 block/blk-mq.c:1016
 blk_mq_end_request+0x26/0x50 block/blk-mq.c:1178
 lo_complete_rq+0x98/0x140 drivers/block/loop.c:314
 blk_complete_reqs block/blk-mq.c:1253 [inline]
 blk_done_softirq+0x77/0xb0 block/blk-mq.c:1258
 handle_softirqs+0xb9/0x280 kernel/softirq.c:622
 run_ksoftirqd+0x1c/0x30 kernel/softirq.c:1076
 smpboot_thread_fn+0x32a/0x510 kernel/smpboot.c:160
 kthread+0x22a/0x280 kernel/kthread.c:436
 ret_from_fork+0x146/0x330 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

value changed: 0x00000145 -> 0x00000146

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 14 Comm: ksoftirqd/0 Tainted: G        W           syzkaller #0 PREEMPT(lazy) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
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