On Tue, Apr 06, 2021 at 06:44:34PM -0700, Michel Lespinasse wrote: > The counter's write side is hooked into the existing mmap locking API: > mmap_write_lock() increments the counter to the next (odd) value, and > mmap_write_unlock() increments it again to the next (even) value. > > The counter's speculative read side is supposed to be used as follows: > > seq = mmap_seq_read_start(mm); > if (seq & 1) > goto fail; > .... speculative handling here .... > if (!mmap_seq_read_check(mm, seq) > goto fail; > > This API guarantees that, if none of the "fail" tests abort > speculative execution, the speculative code section did not run > concurrently with any mmap writer. So this is obviously safe, but it's also super excessive. Any change, anywhere, will invalidate and abort a SPF. Since you make a complete copy of the vma, you could memcmp it in its entirety instead of this.
