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BUG: KCSAN: data-race in cgroup_freezer_migrate_task / handle_mm_fault

write to 0xffff888099bfd498 of 1 bytes by task 25458 on cpu 1:
 mem_cgroup_exit_user_fault include/linux/memcontrol.h:656 [inline]
 handle_mm_fault+0x10c5/0x1af0 mm/memory.c:4636
 do_user_addr_fault+0x380/0x770 arch/x86/mm/fault.c:1372
 handle_page_fault arch/x86/mm/fault.c:1429 [inline]
 exc_page_fault+0xb8/0x330 arch/x86/mm/fault.c:1482
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:538

read to 0xffff888099bfd498 of 1 bytes by task 1 on cpu 0:
 cgroup_freezer_migrate_task+0xa9/0x290 kernel/cgroup/freezer.c:239
 cgroup_migrate_execute+0x2d9/0x8a0 kernel/cgroup/cgroup.c:2432
 cgroup_migrate kernel/cgroup/cgroup.c:2707 [inline]
 cgroup_attach_task+0x1bc/0x340 kernel/cgroup/cgroup.c:2740
 __cgroup1_procs_write+0x1ae/0x220 kernel/cgroup/cgroup-v1.c:519
 cgroup1_procs_write+0x26/0x30 kernel/cgroup/cgroup-v1.c:532
 cgroup_file_write+0x15b/0x2d0 kernel/cgroup/cgroup.c:3697
 kernfs_fop_write+0x207/0x2c0 fs/kernfs/file.c:315
 vfs_write+0x1df/0x6f0 fs/read_write.c:576
 ksys_write+0xce/0x180 fs/read_write.c:631
 __do_sys_write fs/read_write.c:643 [inline]
 __se_sys_write fs/read_write.c:640 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:640
 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: 1 Comm: systemd Not tainted 5.9.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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