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BUG: KCSAN: data-race in data_alloc / prb_reserve

write to 0xffffffff8686a3b8 of 8 bytes by task 4469 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
 br_set_state+0x28c/0x390 net/bridge/br_stp.c:57
 br_stp_disable_port+0x5d/0x150 net/bridge/br_stp_if.c:106
 del_nbp+0xdc/0x790 net/bridge/br_if.c:343
 br_dev_delete+0x4e/0xe0 net/bridge/br_if.c:386
 br_net_exit_rtnl+0xae/0x120 net/bridge/br.c:380
 ops_exit_rtnl_list net/core/net_namespace.c:182 [inline]
 ops_undo_list+0x1b4/0x410 net/core/net_namespace.c:249
 cleanup_net+0x2de/0x4d0 net/core/net_namespace.c:686
 process_one_work kernel/workqueue.c:3238 [inline]
 process_scheduled_works+0x4ce/0x9d0 kernel/workqueue.c:3321
 worker_thread+0x582/0x770 kernel/workqueue.c:3402
 kthread+0x486/0x510 kernel/kthread.c:464
 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 0xffffffff8686a3b8 of 8 bytes by task 12264 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
 set_capacity_and_notify+0x14c/0x1f0 block/genhd.c:93
 loop_set_size+0x2e/0x70 drivers/block/loop.c:210
 loop_configure+0x8d3/0xa50 drivers/block/loop.c:1068
 lo_ioctl+0x559/0x15d0 drivers/block/loop.c:-1
 blkdev_ioctl+0x352/0x440 block/ioctl.c:704
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:907 [inline]
 __se_sys_ioctl+0xce/0x140 fs/ioctl.c:893
 __x64_sys_ioctl+0x43/0x50 fs/ioctl.c:893
 x64_sys_call+0x19a8/0x2fb0 arch/x86/include/generated/asm/syscalls_64.h:17
 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: 0xfffffffffffc3458 -> 0x00000000000618e8

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 12264 Comm: syz.0.2979 Not tainted 6.16.0-rc2-syzkaller-00378-gb67ec639010f #0 PREEMPT(voluntary) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
==================================================================
loop0: detected capacity change from 0 to 256
FAT-fs (loop0): bogus number of FAT sectors
FAT-fs (loop0): Can't find a valid FAT filesystem