For me 1 looks less meaningful than CALLOUT_MPSAFE. If you want to do cleanup, my suggestion is to use Bool argument.

On second thought, may be it is possible to do the following:

1. Remove the second agument, assuming all callouts are mpsafe.
2. Convert non-mpsafe callouts to callout_init_mtx(..., &Giant);
3. Remove special Giant handling from the kern_timeout.c

Some of this may be my fault as I had thought that CALLOUT_MPSAFE is what was supposed to be passed to this.

I think this is fine for now. I think longer term I prefer Gleb's suggestion of requiring non-safe callouts to use callout_init_mtx with &Giant. I'm not sure how many of these callouts still exist. There is one timeout() user left that I still need to get tested, but I hope to remove timeout() very soon.

In D2613#48666, @jhb wrote:

I think longer term I prefer Gleb's suggestion of requiring non-safe callouts to use callout_init_mtx with &Giant.

Yes, I like Gleb's idea, too. I'll commit this patch first and open another revision.