publishing on arxiv a journal paper that is under revision I submitted a paper to an ACM journal, and it's under revision.

Can I publish it in arxiv in meanwhile? because the revision process will take time.
The ACM's open access policy can be found here:
https://www.acm.org/publications/openaccess

As you can read, in principle, you have the right to "self-archive" you manuscript on the ArXiv. However, do check with the journal in question. (I can for example, imagine issues occuring when there is a double blind peer review.)