what size does stat return while write?

On Linux, suppose I do a write() to end of a file, and while this is still completing, from another thread I do a stat-type call on that file (such as fstat() or lstat()). I would expect, that the stat-buffer st_size field during that time, would return between, the previous size of the file before the write(), and, the current size of valid data written already present in the file. So that, if I were to use the st_size to mmap() the file with 0 offset, I will get valid data; perhaps not all the data yet, OK, but at least, all the previous contents of the file, plus perhaps some valid already written data. Is that guaranteed ?