Why exactly don't we want to hold the lock here?

What's an example of a chainevh in the tree? I can't find any uses with a quick look, but I'm probably missing something obvious.

That's because it's set in a macro that's used all over the place, but often takes NULL parameters.
grep -r sys/dev MOD_LOAD: gives a good proxy for what to grep for. Many of them are for DEV_MODULE which is for dev_t not device_t.
Others are for DEVICE_MODULE where the next to last parameter is non NULL.
Some clearly require locking. Others ambiguously require it, but maybe not. Some are safe. I'm thinking for the first round we may want to keep the topo lock over these calls.
.... but some newer ones like nvmf look like they should be called unlocked.
So maybe I need to do a careful audit before proceeding. Today we call them all with Giant locked though.

oh, and things like

moduledata_t icl_soft_data = {
        "icl_soft",
        icl_soft_modevent,
        0
};