[moderation] [v9fs?] KCSAN: data-race in p9_poll_mux / p9_write_work

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 8eefed8f65cc Merge tag 'nfsd-6.18-2' of git://git.kernel.o..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15636704580000
kernel config: https://syzkaller.appspot.com/x/.config?x=d2e0bc0ae94545f7
dashboard link: https://syzkaller.appspot.com/bug?extid=7a8df0239d59eebc5ea9
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
CC: [asma...@codewreck.org eri...@kernel.org linux-...@vger.kernel.org linu...@crudebyte.com lu...@ionkov.net v9...@lists.linux.dev]

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/708e3e1cabf9/disk-8eefed8f.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/f06af46c4069/vmlinux-8eefed8f.xz
kernel image: https://storage.googleapis.com/syzbot-assets/677615feaacf/bzImage-8eefed8f.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+7a8df0...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in p9_poll_mux / p9_write_work

write to 0xffff88811b1eb89c of 4 bytes by task 4333 on cpu 1:
p9_write_work+0x414/0x720 net/9p/trans_fd.c:500
process_one_work kernel/workqueue.c:3263 [inline]
process_scheduled_works+0x4ce/0x9d0 kernel/workqueue.c:3346
worker_thread+0x582/0x770 kernel/workqueue.c:3427
kthread+0x489/0x510 kernel/kthread.c:463
ret_from_fork+0x122/0x1b0 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

read to 0xffff88811b1eb89c of 4 bytes by task 3540 on cpu 0:
p9_poll_mux+0x127/0x2f0 net/9p/trans_fd.c:648
p9_poll_workfn+0xbf/0x120 net/9p/trans_fd.c:1165
process_one_work kernel/workqueue.c:3263 [inline]
process_scheduled_works+0x4ce/0x9d0 kernel/workqueue.c:3346
worker_thread+0x582/0x770 kernel/workqueue.c:3427
kthread+0x489/0x510 kernel/kthread.c:463
ret_from_fork+0x122/0x1b0 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

value changed: 0x00000015 -> 0x00000000

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 3540 Comm: kworker/0:6 Tainted: G W syzkaller #0 PREEMPT(voluntary)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
Workqueue: events p9_poll_workfn
==================================================================


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