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BUG: KCSAN: data-race in _prb_read_valid / prb_reserve

write to 0xffffffff86edd9d8 of 88 bytes by task 4478 on cpu 0:
 prb_reserve+0x6fb/0xaf0 kernel/printk/printk_ringbuffer.c:1693
 vprintk_store+0x54e/0x910 kernel/printk/printk.c:2354
 vprintk_emit+0x1a4/0x610 kernel/printk/printk.c:2455
 vprintk_default+0x26/0x30 kernel/printk/printk.c:2494
 vprintk+0x1d/0x30 kernel/printk/printk_safe.c:82
 _printk+0x79/0xa0 kernel/printk/printk.c:2504
 printk_stack_address arch/x86/kernel/dumpstack.c:70 [inline]
 __show_trace_log_lvl+0x460/0x560 arch/x86/kernel/dumpstack.c:282
 __dump_stack+0x1d/0x30 lib/dump_stack.c:94
 dump_stack_lvl+0x95/0xd0 lib/dump_stack.c:120
 dump_stack+0x15/0x1b lib/dump_stack.c:129
 fail_dump lib/fault-inject.c:73 [inline]
 should_fail_ex+0x262/0x280 lib/fault-inject.c:174
 should_failslab+0x8c/0xb0 mm/failslab.c:46
 slab_pre_alloc_hook mm/slub.c:4638 [inline]
 slab_alloc_node mm/slub.c:4974 [inline]
 __do_kmalloc_node mm/slub.c:5413 [inline]
 __kmalloc_noprof+0xd0/0x4d0 mm/slub.c:5439
 _kmalloc_noprof include/linux/slab.h:995 [inline]
 memcg_init_list_lru_one mm/list_lru.c:468 [inline]
 __memcg_list_lru_alloc+0x288/0x470 mm/list_lru.c:601
 memcg_list_lru_alloc+0x88/0xa0 mm/list_lru.c:628
 __memcg_slab_post_alloc_hook+0x23f/0x8d0 mm/memcontrol.c:3632
 memcg_slab_post_alloc_hook mm/slub.c:2517 [inline]
 slab_post_alloc_hook mm/slub.c:4702 [inline]
 slab_alloc_node mm/slub.c:4996 [inline]
 kmem_cache_alloc_lru_noprof+0x1f4/0x3f0 mm/slub.c:5029
 mqueue_alloc_inode+0x34/0x50 ipc/mqueue.c:501
 alloc_inode+0x40/0x170 fs/inode.c:338
 new_inode+0x1d/0xe0 fs/inode.c:1171
 mqueue_get_inode ipc/mqueue.c:296 [inline]
 mqueue_fill_super+0x89/0x1d0 ipc/mqueue.c:415
 vfs_get_super fs/super.c:1391 [inline]
 get_tree_nodev+0x83/0x100 fs/super.c:1410
 mqueue_get_tree+0x5d/0x80 ipc/mqueue.c:435
 vfs_get_tree+0x57/0x1d0 fs/super.c:1933
 fc_mount fs/namespace.c:1198 [inline]
 fc_mount_longterm+0x18/0xb0 fs/namespace.c:1209
 mq_create_mount ipc/mqueue.c:485 [inline]
 mq_init_ns+0x2db/0x370 ipc/mqueue.c:1630
 create_ipc_ns ipc/namespace.c:73 [inline]
 copy_ipcs+0x26d/0x460 ipc/namespace.c:116
 create_new_namespaces+0x121/0x420 kernel/nsproxy.c:112
 unshare_nsproxy_namespaces+0x115/0x150 kernel/nsproxy.c:234
 ksys_unshare+0x42b/0x710 kernel/fork.c:3295
 __do_sys_unshare kernel/fork.c:3359 [inline]
 __se_sys_unshare kernel/fork.c:3357 [inline]
 __x64_sys_unshare+0x1f/0x30 kernel/fork.c:3357
 x64_sys_call+0x1e33/0x2550 arch/x86/include/generated/asm/syscalls_64.h:273
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x112/0x360 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffffffff86edd9d8 of 8 bytes by task 4127 on cpu 1:
 desc_read kernel/printk/printk_ringbuffer.c:500 [inline]
 prb_first_seq kernel/printk/printk_ringbuffer.c:2017 [inline]
 _prb_read_valid+0x325/0x930 kernel/printk/printk_ringbuffer.c:2174
 prb_read_valid+0x3c/0x60 kernel/printk/printk_ringbuffer.c:2245
 printk_get_next_message+0xea/0x550 kernel/printk/printk.c:3052
 nbcon_emit_next_record+0x4af/0x1bc0 kernel/printk/nbcon.c:1017
 nbcon_emit_one kernel/printk/nbcon.c:1157 [inline]
 nbcon_kthread_func+0x3fd/0x690 kernel/printk/nbcon.c:1271
 kthread+0x223/0x270 kernel/kthread.c:436
 ret_from_fork+0x146/0x330 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

value changed: 0x0000000000002d51 -> 0x0000000000004d51

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 4127 Comm: pr/ttyS0 Not tainted syzkaller #0 PREEMPT(lazy) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
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