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BUG: KCSAN: data-race in handle_userfault / userfaultfd_wake_function

read to 0xffffc90000ebbb40 of 8 bytes by task 10992 on cpu 0:
 list_empty_careful include/linux/list.h:287 [inline]
 handle_userfault+0xe8d/0xfd0 fs/userfaultfd.c:564
 do_anonymous_page mm/memory.c:3035 [inline]
 handle_pte_fault mm/memory.c:3847 [inline]
 __handle_mm_fault+0x2bfe/0x2cb0 mm/memory.c:3973
 handle_mm_fault+0x21b/0x530 mm/memory.c:4010
 faultin_page mm/gup.c:640 [inline]
 __get_user_pages+0x485/0x1160 mm/gup.c:845
 populate_vma_page_range+0xe6/0x100 mm/gup.c:1223
 __mm_populate+0x168/0x2a0 mm/gup.c:1271
 do_mlock+0x19d/0x480 mm/mlock.c:712
 __do_sys_mlock mm/mlock.c:720 [inline]
 __se_sys_mlock mm/mlock.c:718 [inline]
 __x64_sys_mlock+0x3f/0x50 mm/mlock.c:718
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffffc90000ebbb40 of 8 bytes by task 10994 on cpu 1:
 INIT_LIST_HEAD include/linux/list.h:29 [inline]
 list_del_init include/linux/list.h:191 [inline]
 userfaultfd_wake_function fs/userfaultfd.c:140 [inline]
 userfaultfd_wake_function+0x168/0x1d0 fs/userfaultfd.c:106
 __wake_up_common+0x7b/0x180 kernel/sched/wait.c:93
 __wake_up_locked_key+0x16/0x20 kernel/sched/wait.c:157
 userfaultfd_release+0x30e/0x3a0 fs/userfaultfd.c:933
 __fput+0x1e1/0x520 fs/file_table.c:280
 ____fput+0x1f/0x30 fs/file_table.c:313
 task_work_run+0xf6/0x130 kernel/task_work.c:113
 tracehook_notify_resume include/linux/tracehook.h:188 [inline]
 exit_to_usermode_loop+0x2b4/0x2c0 arch/x86/entry/common.c:163
 prepare_exit_to_usermode arch/x86/entry/common.c:194 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:274 [inline]
 do_syscall_64+0x353/0x370 arch/x86/entry/common.c:300
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 10994 Comm: syz-executor.4 Not tainted 5.4.0-rc3+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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