[moderation] [mm?] [fs?] KCSAN: data-race in __wb_update_bandwidth / percpu_counter_add_batch (12)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 9716c086c8e8 Merge tag 'pm-7.1-rc8' of git://git.kernel.or..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13d0b8ae580000
kernel config: https://syzkaller.appspot.com/x/.config?x=f571f22917457cd8
dashboard link: https://syzkaller.appspot.com/bug?extid=44b22b980a89a40cad9a
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/5ba9e6db562e/disk-9716c086.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/b1328421a145/vmlinux-9716c086.xz
kernel image: https://storage.googleapis.com/syzbot-assets/f5d8ba373927/bzImage-9716c086.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+44b22b...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in __wb_update_bandwidth / percpu_counter_add_batch

read-write to 0xffff888101cdd148 of 8 bytes by interrupt on cpu 1:
percpu_counter_add_batch+0x105/0x130 lib/percpu_counter.c:107
wb_stat_mod include/linux/backing-dev.h:66 [inline]
__wb_writeout_add+0x42/0x1b0 mm/page-writeback.c:577
__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
end_buffer_async_write+0x156/0x1f0 fs/buffer.c:415
end_bio_bh_io_sync+0x79/0xa0 fs/buffer.c:2673
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
__do_softirq kernel/softirq.c:656 [inline]
invoke_softirq kernel/softirq.c:496 [inline]
__irq_exit_rcu+0x42/0xd0 kernel/softirq.c:735
instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1061 [inline]
sysvec_apic_timer_interrupt+0x74/0x80 arch/x86/kernel/apic/apic.c:1061
asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:697
__preempt_count_dec_and_test arch/x86/include/asm/preempt.h:95 [inline]
__raw_spin_unlock_irqrestore include/linux/spinlock_api_smp.h:179 [inline]
_raw_spin_unlock_irqrestore+0x1a/0x30 kernel/locking/spinlock.c:198
spin_unlock_irqrestore include/linux/spinlock.h:408 [inline]
lruvec_unlock_irqrestore include/linux/memcontrol.h:1492 [inline]
folio_batch_move_lru+0x246/0x290 mm/swap.c:178
__folio_batch_add_and_move mm/swap.c:196 [inline]
folio_add_lru+0x14f/0x1f0 mm/swap.c:517
shmem_alloc_and_add_folio mm/shmem.c:2034 [inline]
shmem_get_folio_gfp+0x7b0/0xd60 mm/shmem.c:2564
shmem_get_folio mm/shmem.c:2670 [inline]
shmem_write_begin+0xfc/0x1f0 mm/shmem.c:3303
generic_perform_write+0x183/0x490 mm/filemap.c:4325
shmem_file_write_iter+0xc5/0xf0 mm/shmem.c:3478
lo_rw_aio+0x67d/0x730 drivers/block/loop.c:-1
do_req_filebacked drivers/block/loop.c:-1 [inline]
loop_handle_cmd drivers/block/loop.c:1925 [inline]
loop_process_work+0x56c/0xac0 drivers/block/loop.c:1960
loop_workfn+0x31/0x40 drivers/block/loop.c:1984
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 0xffff888101cdd148 of 8 bytes by task 15115 on cpu 0:
percpu_counter_read include/linux/percpu_counter.h:110 [inline]
__wb_update_bandwidth+0x98/0x5d0 mm/page-writeback.c:1501
wb_update_bandwidth mm/page-writeback.c:1528 [inline]
do_writepages+0x2dd/0x310 mm/page-writeback.c:2594
filemap_writeback mm/filemap.c:388 [inline]
filemap_fdatawrite_range mm/filemap.c:413 [inline]
filemap_write_and_wait_range+0x166/0x370 mm/filemap.c:685
filemap_write_and_wait include/linux/pagemap.h:65 [inline]
sync_blockdev block/bdev.c:276 [inline]
bdev_release+0xeb/0x3d0 block/bdev.c:1134
blkdev_release+0x15/0x20 block/fops.c:705
__fput+0x29b/0x630 fs/file_table.c:510
____fput+0x1c/0x30 fs/file_table.c:538
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+0x211/0x820 kernel/entry/common.c:98
__exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline]
syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:230 [inline]
syscall_exit_to_user_mode include/linux/entry-common.h:318 [inline]
do_syscall_64+0x232/0x3b0 arch/x86/entry/syscall_64.c:100
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x00000000000035b0 -> 0x00000000000035c0

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 15115 Comm: syz.5.4680 Not tainted syzkaller #0 PREEMPT(lazy)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
==================================================================


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