KCSAN: data-race in p9_poll_mux / p9_write

Kernel	Title	Rank 🛈	Repro	Cause bisect	Fix bisect	Count	Last	Reported	Patched	Status
upstream	KCSAN: data-race in p9_poll_mux / p9_write_work (2) race:benign prio:low v9fs	6				1	52d	52d	0/29	moderation: reported on 2026/06/26 23:49

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