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BUG: KCSAN: data-race in exit_signals / get_task_mm

read-write to 0xffff88802dc9f02c of 4 bytes by task 31141 on cpu 1:
 exit_signals+0x102/0x530 kernel/signal.c:2862
 do_exit+0x15d/0x1630 kernel/exit.c:779
 do_group_exit+0x16f/0x170 kernel/exit.c:920
 __do_sys_exit_group+0xb/0x10 kernel/exit.c:931
 __se_sys_exit_group+0x5/0x10 kernel/exit.c:929
 __x64_sys_exit_group+0x16/0x20 kernel/exit.c:929
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88802dc9f02c of 4 bytes by task 31146 on cpu 0:
 get_task_mm+0x3e/0x90 kernel/fork.c:1213
 kernel_migrate_pages+0x255/0x350 mm/mempolicy.c:1569
 __do_sys_migrate_pages mm/mempolicy.c:1596 [inline]
 __se_sys_migrate_pages mm/mempolicy.c:1592 [inline]
 __x64_sys_migrate_pages+0x50/0x60 mm/mempolicy.c:1592
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 31146 Comm: syz-executor.0 Not tainted 5.11.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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