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BUG: KCSAN: data-race in p9_poll_workfn / p9_write_work

write to 0xffff888123b6889c of 4 bytes by task 6424 on cpu 0:
 p9_write_work+0x42c/0x750 net/9p/trans_fd.c:499
 process_one_work kernel/workqueue.c:3254 [inline]
 process_scheduled_works+0x465/0x990 kernel/workqueue.c:3335
 worker_thread+0x526/0x730 kernel/workqueue.c:3416
 kthread+0x1d1/0x210 kernel/kthread.c:388
 ret_from_fork+0x4b/0x60 arch/x86/kernel/process.c:147
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:243

read to 0xffff888123b6889c of 4 bytes by task 6452 on cpu 1:
 p9_poll_mux net/9p/trans_fd.c:647 [inline]
 p9_poll_workfn+0x2bf/0x420 net/9p/trans_fd.c:1178
 process_one_work kernel/workqueue.c:3254 [inline]
 process_scheduled_works+0x465/0x990 kernel/workqueue.c:3335
 worker_thread+0x526/0x730 kernel/workqueue.c:3416
 kthread+0x1d1/0x210 kernel/kthread.c:388
 ret_from_fork+0x4b/0x60 arch/x86/kernel/process.c:147
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:243

value changed: 0x00000015 -> 0x00000000

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 6452 Comm: kworker/1:13 Not tainted 6.9.0-rc1-syzkaller-00011-g498e47cd1d1f #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/29/2024
Workqueue: events p9_poll_workfn
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