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BUG: KCSAN: data-race in mq_clear_sbinfo / test_keyed_super

write to 0xffff888166cc3b70 of 8 bytes by task 25440 on cpu 0:
 mq_clear_sbinfo+0x35/0x50 ipc/mqueue.c:1694
 put_ipc_ns+0x28/0xb0 ipc/namespace.c:168
 free_nsproxy+0xb8/0x310 kernel/nsproxy.c:195
 put_nsproxy include/linux/nsproxy.h:105 [inline]
 switch_task_namespaces kernel/nsproxy.c:249 [inline]
 exit_task_namespaces+0xa4/0xc0 kernel/nsproxy.c:254
 do_exit+0x46f/0x1560 kernel/exit.c:824
 do_group_exit+0xa4/0x1a0 kernel/exit.c:922
 __do_sys_exit_group+0xb/0x10 kernel/exit.c:933
 __se_sys_exit_group+0x5/0x10 kernel/exit.c:931
 __x64_sys_exit_group+0x16/0x20 kernel/exit.c:931
 do_syscall_64+0x34/0x50 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff888166cc3b70 of 8 bytes by task 25434 on cpu 1:
 test_keyed_super+0x1a/0x40 fs/super.c:1077
 sget_fc+0xe8/0x4e0 fs/super.c:524
 vfs_get_super fs/super.c:1134 [inline]
 get_tree_keyed+0x3f/0x120 fs/super.c:1195
 mqueue_get_tree+0x32/0x40 ipc/mqueue.c:427
 vfs_get_tree+0x4a/0x1a0 fs/super.c:1497
 fc_mount+0x12/0x60 fs/namespace.c:993
 mq_create_mount ipc/mqueue.c:470 [inline]
 mq_init_ns+0x26d/0x310 ipc/mqueue.c:1685
 create_ipc_ns ipc/namespace.c:58 [inline]
 copy_ipcs+0x211/0x360 ipc/namespace.c:84
 create_new_namespaces+0x137/0x560 kernel/nsproxy.c:90
 copy_namespaces+0x111/0x160 kernel/nsproxy.c:178
 copy_process+0x1273/0x2e20 kernel/fork.c:2116
 kernel_clone+0x14b/0x680 kernel/fork.c:2500
 __do_sys_clone kernel/fork.c:2617 [inline]
 __se_sys_clone kernel/fork.c:2601 [inline]
 __x64_sys_clone+0xb4/0xe0 kernel/fork.c:2601
 do_syscall_64+0x34/0x50 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 25434 Comm: syz-executor.4 Not tainted 5.12.0-rc8-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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