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BUG: KCSAN: data-race in data_alloc / prb_reserve

write to 0xffffffff86872698 of 8 bytes by task 21546 on cpu 1:
 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
 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
 stop_sync_thread+0x255/0x3f0 net/netfilter/ipvs/ip_vs_sync.c:1966
 do_ip_vs_set_ctl+0x2f1/0x8c0 net/netfilter/ipvs/ip_vs_ctl.c:2735
 nf_setsockopt+0x199/0x1b0 net/netfilter/nf_sockopt.c:101
 ip_setsockopt+0x102/0x110 net/ipv4/ip_sockglue.c:1424
 tcp_setsockopt+0x95/0xb0 net/ipv4/tcp.c:4108
 sock_common_setsockopt+0x66/0x80 net/core/sock.c:3944
 smc_setsockopt+0x180/0x750 net/smc/af_smc.c:3086
 do_sock_setsockopt net/socket.c:2296 [inline]
 __sys_setsockopt+0x181/0x200 net/socket.c:2321
 __do_sys_setsockopt net/socket.c:2327 [inline]
 __se_sys_setsockopt net/socket.c:2324 [inline]
 __x64_sys_setsockopt+0x64/0x80 net/socket.c:2324
 x64_sys_call+0x2bd5/0x2fb0 arch/x86/include/generated/asm/syscalls_64.h:55
 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 0xffffffff86872698 of 8 bytes by task 21566 on cpu 0:
 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
 sync_thread_master+0xb8/0x8b0 net/netfilter/ipvs/ip_vs_sync.c:1658
 kthread+0x486/0x510 kernel/kthread.c:464
 ret_from_fork+0xdd/0x150 arch/x86/kernel/process.c:148
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

value changed: 0x000000000007ba48 -> 0x00000000001144d0

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 21566 Comm: ipvs-m:202:0 Not tainted 6.16.0-rc2-syzkaller-00082-gfb4d33ab452e #0 PREEMPT(voluntary) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
==================================================================
IPVS: sync thread started: state = MASTER, mcast_ifn = veth0_virt_wifi, syncid = 33554432, id = 0