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BUG: KCSAN: data-race in exit_signals / mm_update_next_owner

write to 0xffff88802e7b902c of 4 bytes by task 24537 on cpu 0:
 exit_signals+0x118/0x550 kernel/signal.c:2873
 do_exit+0x197/0x15e0 kernel/exit.c:781
 call_usermodehelper_exec_async+0x246/0x250 kernel/umh.c:123
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

read to 0xffff88802e7b902c of 4 bytes by task 24538 on cpu 1:
 mm_update_next_owner+0x1d9/0x4b0 kernel/exit.c:388
 exit_mm+0x33c/0x430 kernel/exit.c:500
 do_exit+0x3fc/0x15e0 kernel/exit.c:812
 do_group_exit+0x17d/0x180 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+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 24538 Comm: syz-executor.0 Not tainted 5.12.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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