Re: TLB flushes on fixmap changes

[Peter Zijlstra]

On Sat, Aug 25, 2018 at 09:21:22PM -0700, Andy Lutomirski wrote:
> I just re-read text_poke().  It's, um, horrible.  Not only is the
> implementation overcomplicated and probably buggy, but it's SLOOOOOW.
> It's totally the wrong API -- poking one instruction at a time
> basically can't be efficient on x86.  The API should either poke lots
> of instructions at once or should be text_poke_begin(); ...;
> text_poke_end();.

I don't think anybody ever cared about performance here. Only
correctness. That whole text_poke_bp() thing is entirely tricky.

FWIW, before text_poke_bp(), text_poke() would only be used from
stop_machine, so all the other CPUs would be stuck busy-waiting with
IRQs disabled. These days, yeah, that's lots more dodgy, but yes
text_mutex should be serializing all that.

And on that, I so hate comments like: "must be called under foo_mutex",
we have lockdep_assert_held() for that.