[moderation] [block?] KCSAN: data-race in blk_mq_start_request / bt_for_each (4)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 08b06c30a445 Merge tag 'v6.17-rc4-ksmbd-fix' of git://git...
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14971a42580000
kernel config: https://syzkaller.appspot.com/x/.config?x=52ae2e92d7a28724
dashboard link: https://syzkaller.appspot.com/bug?extid=cef23fee9615d8ef7a5f
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
CC: [ax...@kernel.dk linux...@vger.kernel.org linux-...@vger.kernel.org]

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/c5608ce187df/disk-08b06c30.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/db0b36f6dd67/vmlinux-08b06c30.xz
kernel image: https://storage.googleapis.com/syzbot-assets/6113df78d81d/bzImage-08b06c30.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+cef23f...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in blk_mq_start_request / bt_for_each

write to 0xffff888102469b78 of 8 bytes by task 7077 on cpu 0:
blk_mq_start_request+0x1e1/0x3d0 block/blk-mq.c:1359
loop_queue_rq+0x43/0x680 drivers/block/loop.c:1856
__blk_mq_issue_directly block/blk-mq.c:2695 [inline]
blk_mq_request_issue_directly+0x1e6/0x380 block/blk-mq.c:2782
blk_mq_issue_direct+0x19d/0x530 block/blk-mq.c:2803
blk_mq_dispatch_queue_requests+0x1fc/0x350 block/blk-mq.c:2878
blk_mq_flush_plug_list+0x2b6/0x330 block/blk-mq.c:2961
__blk_flush_plug+0x222/0x2a0 block/blk-core.c:1220
blk_finish_plug block/blk-core.c:1247 [inline]
__submit_bio+0x2fc/0x4d0 block/blk-core.c:649
__submit_bio_noacct_mq block/blk-core.c:722 [inline]
submit_bio_noacct_nocheck+0x208/0x6a0 block/blk-core.c:751
submit_bio_noacct+0x6ed/0x920 block/blk-core.c:874
submit_bio+0x2a6/0x2c0 block/blk-core.c:916
submit_bh_wbc+0x2e0/0x320 fs/buffer.c:2824
submit_bh fs/buffer.c:2829 [inline]
__sync_dirty_buffer+0x16b/0x230 fs/buffer.c:2867
sync_dirty_buffer+0x1a/0x30 fs/buffer.c:2880
ext4_update_primary_sb fs/ext4/ioctl.c:88 [inline]
ext4_update_superblocks_fn+0x307/0xaa0 fs/ext4/ioctl.c:218
ext4_update_overhead+0x135/0x160 fs/ext4/ioctl.c:1721
ext4_fill_super+0x2e05/0x35d0 fs/ext4/super.c:5749
get_tree_bdev_flags+0x28e/0x300 fs/super.c:1692
get_tree_bdev+0x1f/0x30 fs/super.c:1715
ext4_get_tree+0x1c/0x30 fs/ext4/super.c:5760
vfs_get_tree+0x57/0x1d0 fs/super.c:1815
do_new_mount+0x207/0x5e0 fs/namespace.c:3808
path_mount+0x4a4/0xb20 fs/namespace.c:4123
do_mount fs/namespace.c:4136 [inline]
__do_sys_mount fs/namespace.c:4347 [inline]
__se_sys_mount+0x28f/0x2e0 fs/namespace.c:4324
__x64_sys_mount+0x67/0x80 fs/namespace.c:4324
x64_sys_call+0x2b4d/0x2ff0 arch/x86/include/generated/asm/syscalls_64.h:166
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd2/0x200 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888102469b78 of 8 bytes by task 47 on cpu 1:
blk_mq_find_and_get_req block/blk-mq-tag.c:259 [inline]
bt_iter block/blk-mq-tag.c:287 [inline]
__sbitmap_for_each_set include/linux/sbitmap.h:269 [inline]
sbitmap_for_each_set include/linux/sbitmap.h:290 [inline]
bt_for_each+0x274/0x470 block/blk-mq-tag.c:324
blk_mq_queue_tag_busy_iter+0x1ba/0x330 block/blk-mq-tag.c:533
blk_mq_timeout_work+0xc2/0x340 block/blk-mq.c:1734
process_one_work kernel/workqueue.c:3236 [inline]
process_scheduled_works+0x4ce/0x9d0 kernel/workqueue.c:3319
worker_thread+0x582/0x770 kernel/workqueue.c:3400
kthread+0x486/0x510 kernel/kthread.c:463
ret_from_fork+0xda/0x150 arch/x86/kernel/process.c:148
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

value changed: 0xffff888101245400 -> 0xffff88810252a680

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 47 Comm: kworker/1:1H Not tainted syzkaller #0 PREEMPT(voluntary)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
Workqueue: kblockd blk_mq_timeout_work
==================================================================


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