Re: [PATCH v1 2/2] epoll: Use __user_write_access_begin() and unsafe_put_user() in epoll_put_uevent().

Yeah, clac/stac are horrenously expensive on Zen 2. I think they are
microcoded - and not the fast kind - so effectively serializing.

AIUI, Zen5 is the first uarch capable of renaming the AC flag.

This make STAC/CLAC "lfence-like" in practice on older CPUs, because of
an implementation constraint of not being able to execute speculatively.

~Andrew