Re: [RFC PATCH 1/7] static_call: get rid of static_call_cond()

Peter Zijlstra
2021-11-09T19:22:48Z

On Tue, Nov 09, 2021 at 07:41:03PM +0100, Ard Biesheuvel wrote:
> On Tue, 9 Nov 2021 at 19:38, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Tue, Nov 09, 2021 at 05:45:43PM +0100, Ard Biesheuvel wrote:
> > > The main reason for the existence of static_call_cond() seems to be that
> > > in theory, when using the generic implementation of static calls, it may
> > > be possible for a compiler to elide the indirect call entirely if the
> > > target is NULL, while still guaranteeing that all side effects of
> > > argument evaluation occur as expected.
> > >
> > > This is rather optimistic: as documented by an existing code comment,
> > > both GCC and Clang (version 10) get this wrong, and even if they ever
> > > get it right, this is far too subtle to rely on for a code path that is
> > > expected to be used only by the 'remaining' architectures once all the
> > > best supported ones implement either the out-of-line or inline optimized
> > > variety of static calls.
> > >
> > > Given that having static_call_cond() clutters up the API, and puts the
> > > burden on the caller to go and check what kind of static call they are
> > > dealing with, let's just get rid of the distinction.
> >
> > No, static_call_cond() signifies the function can be NULL. Both gcc and
> > clang generate correct (but wildly ineffecient) code for this. Without
> > static_call_cond() the generic implementation will do a NULL deref.
> >
> > That is, static_call_cond() does properly encapuslate:
> >
> > func = READ_ONCE(key.func);
> > if (func)
> > func(ARGS);
> >
> > You can't take that out.
>
> I actually address that in the patch.
>
> AIUI, the compiler generates an indirect call to __static_call_nop(),
> right? So why not simply set .func to the address of
> __static_call_nop() when NULL is passed to update / the initializer?

Ooh, lemme go have a proper look then.