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BUG: KCSAN: data-race in exit_signals / get_task_mm

write to 0xffff8880a6799024 of 4 bytes by task 30329 on cpu 0:
 exit_signals+0x124/0x580 kernel/signal.c:2831
 do_exit+0x19f/0x16e0 kernel/exit.c:761
 do_group_exit+0x17a/0x180 kernel/exit.c:903
 __do_sys_exit_group+0xb/0x10 kernel/exit.c:914
 __se_sys_exit_group+0x5/0x10 kernel/exit.c:912
 __x64_sys_exit_group+0x16/0x20 kernel/exit.c:912
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880a6799024 of 4 bytes by task 30331 on cpu 1:
 get_task_mm+0x3e/0x90 kernel/fork.c:1223
 proc_pid_status+0x35/0x1330 fs/proc/array.c:406
 proc_single_show+0x84/0x100 fs/proc/base.c:766
 seq_read+0x2ee/0x900 fs/seq_file.c:208
 do_loop_readv_writev fs/read_write.c:734 [inline]
 do_iter_read+0x2fe/0x480 fs/read_write.c:955
 vfs_readv fs/read_write.c:1073 [inline]
 do_preadv+0x13b/0x230 fs/read_write.c:1165
 __do_sys_preadv fs/read_write.c:1215 [inline]
 __se_sys_preadv fs/read_write.c:1210 [inline]
 __x64_sys_preadv+0x54/0x60 fs/read_write.c:1210
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 30331 Comm: syz-executor.5 Not tainted 5.8.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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