Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64

[Linus Torvalds]

On Thu, Nov 29, 2018 at 9:02 AM Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> >
> > - just restart the instruction (with the suggested "ptregs->rip --")
> >
> > - to avoid any "oh, we're not making progress" issues, just fix the
> > instruction yourself to be the right call, by looking it up in the
> > "what needs to be fixed" tables.
>
> I thought that too.  I think it deadlocks. CPU A does text_poke_bp().  CPU B is waiting for a spinlock with IRQs off.  CPU C holds the spinlock and hits the int3.  The int3 never goes away because CPU A is waiting for CPU B to handle the sync_core IPI.
>
> Or do you think we can avoid the IPI while the int3 is there?

I'm handwaving and thinking that CPU C that hits the int3 can just fix
up the instruction directly in its own caches, and return.

Yes, it does what he "text_poke" *will* do (so now the instruction
gets rewritten _twice_), but who cares? It's idempotent.

And no, I don't have code, just "maybe some handwaving like this"

               Linus