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BUG: KCSAN: data-race in cgroup_procs_write_start / handle_mm_fault

write to 0xffff8881024eb4c0 of 2 bytes by task 8470 on cpu 0:
 mem_cgroup_enter_user_fault include/linux/memcontrol.h:653 [inline]
 handle_mm_fault+0x1a7/0x1770 mm/memory.c:4602
 do_user_addr_fault+0x33e/0x730 arch/x86/mm/fault.c:1372
 handle_page_fault arch/x86/mm/fault.c:1429 [inline]
 exc_page_fault+0x91/0x290 arch/x86/mm/fault.c:1485
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:583

read to 0xffff8881024eb4c0 of 2 bytes by task 1 on cpu 1:
 cgroup_procs_write_start+0x13c/0x270 kernel/cgroup/cgroup.c:2795
 __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+0x173/0x2f0 kernel/cgroup/cgroup.c:3699
 kernfs_fop_write+0x207/0x2c0 fs/kernfs/file.c:315
 vfs_write+0x21a/0x7c0 fs/read_write.c:603
 ksys_write+0xce/0x180 fs/read_write.c:658
 __do_sys_write fs/read_write.c:670 [inline]
 __se_sys_write fs/read_write.c:667 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:667
 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 5.10.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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