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BUG: KCSAN: data-race in fprop_reflect_period_percpu / fprop_reflect_period_percpu

write to 0xffff888100b159c8 of 4 bytes by task 5216 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:2108 [inline]
 domain_over_bg_thresh mm/page-writeback.c:2122 [inline]
 wb_over_bg_thresh+0x222/0x530 mm/page-writeback.c:2143
 wb_check_background_flush fs/fs-writeback.c:2301 [inline]
 wb_do_writeback fs/fs-writeback.c:2399 [inline]
 wb_workfn+0x690/0x970 fs/fs-writeback.c:2426
 process_one_work kernel/workqueue.c:3276 [inline]
 process_scheduled_works+0x513/0xa10 kernel/workqueue.c:3359
 worker_thread+0x58a/0x780 kernel/workqueue.c:3440
 kthread+0x22a/0x280 kernel/kthread.c:436
 ret_from_fork+0x150/0x360 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

read to 0xffff888100b159c8 of 4 bytes by task 6848 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:2953
 folio_end_writeback_no_dropbehind+0x6d/0x1d0 mm/filemap.c:1666
 folio_end_writeback+0x1c/0x70 mm/filemap.c:1692
 __block_write_full_folio+0x5f3/0x8f0 fs/buffer.c:1946
 block_write_full_folio+0x2c1/0x2e0 fs/buffer.c:-1
 blkdev_writepages+0x88/0x100 block/fops.c:486
 do_writepages+0x1c6/0x310 mm/page-writeback.c:2554
 filemap_writeback mm/filemap.c:387 [inline]
 filemap_flush_range mm/filemap.c:436 [inline]
 filemap_flush+0xee/0x130 mm/filemap.c:451
 sync_blockdev_nowait+0x29/0x40 block/bdev.c:264
 sync_filesystem+0xe2/0x190 fs/sync.c:60
 generic_shutdown_super+0x4c/0x220 fs/super.c:625
 kill_block_super+0x2a/0x70 fs/super.c:1725
 ext4_kill_sb+0x42/0x80 fs/ext4/super.c:7484
 deactivate_locked_super+0x75/0x1c0 fs/super.c:476
 deactivate_super+0x97/0xa0 fs/super.c:509
 cleanup_mnt+0x2bb/0x330 fs/namespace.c:1312
 __cleanup_mnt+0x19/0x20 fs/namespace.c:1319
 task_work_run+0x130/0x1a0 kernel/task_work.c:233
 resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
 __exit_to_user_mode_loop kernel/entry/common.c:67 [inline]
 exit_to_user_mode_loop+0x1f4/0x6f0 kernel/entry/common.c:98
 __exit_to_user_mode_prepare include/linux/irq-entry-common.h:226 [inline]
 syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:256 [inline]
 syscall_exit_to_user_mode include/linux/entry-common.h:325 [inline]
 do_syscall_64+0x249/0x370 arch/x86/entry/syscall_64.c:100
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x0000002b -> 0x0000002c

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 6848 Comm: syz-executor Tainted: G        W           syzkaller #0 PREEMPT(full) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
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EXT4-fs (loop7): unmounting filesystem 76b65be2-f6da-4727-8c75-0525a5b65a09.