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BUG: KCSAN: data-race in data_alloc / prb_reserve

write to 0xffffffff8687e940 of 8 bytes by task 6413 on cpu 0:
 data_alloc+0x27d/0x2b0 kernel/printk/printk_ringbuffer.c:1096
 prb_reserve+0x808/0xaf0 kernel/printk/printk_ringbuffer.c:1669
 vprintk_store+0x56d/0x860 kernel/printk/printk.c:2326
 vprintk_emit+0x178/0x650 kernel/printk/printk.c:2426
 dev_vprintk_emit+0x242/0x2a0 drivers/base/core.c:4914
 dev_printk_emit+0x84/0xb0 drivers/base/core.c:4925
 __netdev_printk+0x35c/0x3e0 net/core/dev.c:12598
 netdev_info+0x9b/0xd0 net/core/dev.c:12653
 nsim_udp_tunnel_set_port+0x13b/0x160 drivers/net/netdevsim/udp_tunnels.c:31
 udp_tunnel_nic_device_sync_one net/ipv4/udp_tunnel_nic.c:-1 [inline]
 udp_tunnel_nic_device_sync_by_port net/ipv4/udp_tunnel_nic.c:249 [inline]
 __udp_tunnel_nic_device_sync+0x567/0x9c0 net/ipv4/udp_tunnel_nic.c:292
 udp_tunnel_nic_device_sync_work+0x5d/0x5f0 net/ipv4/udp_tunnel_nic.c:740
 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

read to 0xffffffff8687e940 of 8 bytes by task 16431 on cpu 1:
 desc_read kernel/printk/printk_ringbuffer.c:482 [inline]
 desc_push_tail kernel/printk/printk_ringbuffer.c:778 [inline]
 desc_reserve kernel/printk/printk_ringbuffer.c:924 [inline]
 prb_reserve+0x221/0xaf0 kernel/printk/printk_ringbuffer.c:1619
 vprintk_store+0x56d/0x860 kernel/printk/printk.c:2326
 vprintk_emit+0x178/0x650 kernel/printk/printk.c:2426
 vprintk_default+0x26/0x30 kernel/printk/printk.c:2465
 vprintk+0x1d/0x30 kernel/printk/printk_safe.c:82
 _printk+0x79/0xa0 kernel/printk/printk.c:2475
 p9_fd_create+0x20c/0x280 net/9p/trans_fd.c:1093
 p9_client_create+0x60b/0xbc0 net/9p/client.c:1017
 v9fs_session_init+0xf7/0xde0 fs/9p/v9fs.c:410
 v9fs_mount+0x67/0x5c0 fs/9p/vfs_super.c:122
 legacy_get_tree+0x75/0xd0 fs/fs_context.c:666
 vfs_get_tree+0x54/0x1d0 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

value changed: 0x0000000000006d30 -> 0x00000000000af170

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 16431 Comm: syz.2.4176 Not tainted syzkaller #0 PREEMPT(voluntary) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
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9pnet_fd: Insufficient options for proto=fd