static int ext4_dax_pfn_mkwrite(struct vm_area_struct *vma,
				struct vm_fault *vmf)
{
	struct inode *inode = file_inode(vma->vm_file);
	struct super_block *sb = inode->i_sb;
	int ret = VM_FAULT_NOPAGE;
	loff_t size;

	sb_start_pagefault(sb);
	file_update_time(vma->vm_file);
	down_read(&EXT4_I(inode)->i_mmap_sem);
	size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;
	if (vmf->pgoff >= size)
		ret = VM_FAULT_SIGBUS;
	up_read(&EXT4_I(inode)->i_mmap_sem);
	sb_end_pagefault(sb);

	return ret;
}
The program contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently.