Lockless file reading

[Timo Sirainen <tss@iki.fi>]

(Maybe a bit off topic, but I couldn't get answers elsewhere and it is
about kernel behaviour)

The question is what can happen if I read() a file that's being
simultaneously updated by a write() in another process? If I'm writing
123 over XXX, is it possible that read() returns 1X3 in some conditions?
Is the behaviour filesystem specific? Any idea about other operating
systems?

I'm thinking about implementing lockless file reads that work like:

void write_data(int fd, off_t offset, void *data, size_t size) {
  lock_file(fd);
  pwrite(fd, data, size, offset); // or writev() or copy+single pwrite()
  pwrite(fd, data, size, offset + size);
  unlock_file(fd);
}

void read_data(int fd, off_t offset, void *data, size_t size) {
  unsigned char buf{size*2];
  for (;;) {
    pread(fd, buf, size*2, offset); // or shared mmap()ed access
    if (memcmp(buf, buf+size, size) == 0) break;
    usleep(10);
  }
  memcpy(data, buf, size);
}

The only case when I see that it would break is if we write "1212" but
pread() sees "1X1X" or "X2X2" there.