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BUG: KCSAN: data-race in cgroup_procs_write_start / handle_mm_fault

write to 0xffff8880a34214d8 of 1 bytes by task 10436 on cpu 0:
 mem_cgroup_enter_user_fault include/linux/memcontrol.h:650 [inline]
 handle_mm_fault+0x143/0x1af0 mm/memory.c:4449
 do_user_addr_fault+0x380/0x770 arch/x86/mm/fault.c:1294
 handle_page_fault arch/x86/mm/fault.c:1351 [inline]
 exc_page_fault+0xb8/0x330 arch/x86/mm/fault.c:1404
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:538

read to 0xffff8880a34214d8 of 1 bytes by task 1 on cpu 1:
 cgroup_procs_write_start+0x13c/0x270 kernel/cgroup/cgroup.c:2796
 __cgroup1_procs_write+0x5d/0x220 kernel/cgroup/cgroup-v1.c:500
 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: 1 PID: 1 Comm: systemd Not tainted 5.8.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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