Re: [PATCH 10/15] static_call: Add basic static call infrastructure

[Ard Biesheuvel]

On Fri, 7 Jun 2019 at 10:29, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Thu, Jun 06, 2019 at 10:44:23PM +0000, Nadav Amit wrote:
> > > + * Usage example:
> > > + *
> > > + *   # Start with the following functions (with identical prototypes):
> > > + *   int func_a(int arg1, int arg2);
> > > + *   int func_b(int arg1, int arg2);
> > > + *
> > > + *   # Define a 'my_key' reference, associated with func_a() by default
> > > + *   DEFINE_STATIC_CALL(my_key, func_a);
> > > + *
> > > + *   # Call func_a()
> > > + *   static_call(my_key, arg1, arg2);
> > > + *
> > > + *   # Update 'my_key' to point to func_b()
> > > + *   static_call_update(my_key, func_b);
> > > + *
> > > + *   # Call func_b()
> > > + *   static_call(my_key, arg1, arg2);
> >
> > I think that this calling interface is not very intuitive.
>
> Yeah, it is somewhat unfortunate..
>

Another thing I brought up at the time is that it would be useful to
have the ability to 'reset' a static call to its default target. E.g.,
for crypto modules that implement an accelerated version of a library
interface, removing the module should revert those call sites back to
the original target, without putting a disproportionate burden on the
module itself to implement the logic to support this.


> > I understand that
> > the macros/objtool cannot allow the calling interface to be completely
> > transparent (as compiler plugin could). But, can the macros be used to
> > paste the key with the âstatic_callâ? I think that having something like:
> >
> >   static_call__func(arg1, arg2)
> >
> > Is more readable than
> >
> >   static_call(func, arg1, arg2)
>
> Doesn't really make it much better for me; I think I'd prefer to switch
> to the GCC plugin scheme over this.  ISTR there being some propotypes
> there, but I couldn't quickly locate them.
>

I implemented the GCC plugin here

https://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git/log/?h=static-calls

but IIRC, all it does is annotate call sites exactly how objtool does it.

> > > +}
> > > +
> > > +#define static_call_update(key, func)                                      \
> > > +({                                                                 \
> > > +   BUILD_BUG_ON(!__same_type(func, STATIC_CALL_TRAMP(key)));       \
> > > +   __static_call_update(&key, func);                               \
> > > +})
> >
> > Is this safe against concurrent module removal?
>
> It is for CONFIG_MODULE=n :-)