[moderation] [fs?] [mm?] KCSAN: data-race in fprop_reflect_period_percpu / fprop_reflect_period_percpu (10)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 2b414a95b8f7 Merge tag 's390-7.1-5' of git://git.kernel.or..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1589b3d2580000
kernel config: https://syzkaller.appspot.com/x/.config?x=f571f22917457cd8
dashboard link: https://syzkaller.appspot.com/bug?extid=57fb656a77789edaf68c
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
CC: [ak...@linux-foundation.org ja...@suse.cz linux-...@vger.kernel.org linux-...@vger.kernel.org linu...@kvack.org wi...@infradead.org]

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/a2bc12f628f8/disk-2b414a95.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/97ba8f79b58e/vmlinux-2b414a95.xz
kernel image: https://storage.googleapis.com/syzbot-assets/b78d4c1c5145/bzImage-2b414a95.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+57fb65...@syzkaller.appspotmail.com

==================================================================
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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