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

Josh Poimboeuf
2018-11-30T16:27:20Z

On Thu, Nov 29, 2018 at 03:04:20PM -0800, Linus Torvalds wrote:
> On Thu, Nov 29, 2018 at 12:25 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> >
> > On Thu, Nov 29, 2018 at 11:27:00AM -0800, Andy Lutomirski wrote:
> > >
> > > I propose a different solution:
> > >
> > > As in this patch set, we have a direct and an indirect version. The
> > > indirect version remains exactly the same as in this patch set. The
> > > direct version just only does the patching when all seems well: the
> > > call instruction needs to be 0xe8, and we only do it when the thing
> > > doesn't cross a cache line. Does that work? In the rare case where
> > > the compiler generates something other than 0xe8 or crosses a cache
> > > line, then the thing just remains as a call to the out of line jmp
> > > trampoline. Does that seem reasonable? It's a very minor change to
> > > the patch set.
> >
> > Maybe that would be ok. If my math is right, we would use the
> > out-of-line version almost 5% of the time due to cache misalignment of
> > the address.
>
> Note that I don't think cache-line alignment is necessarily sufficient.
>
> The I$ fetch from the cacheline can happen in smaller chunks, because
> the bus between the I$ and the instruction decode isn't a full
> cacheline (well, it is _now_ in modern big cores, but it hasn't always
> been).
>
> So even if the cacheline is updated atomically, I could imagine seeing
> a partial fetch from the I$ (old values) and then a second partial
> fetch (new values).
>
> It would be interesting to know what the exact fetch rules are.

I've been doing some cross-modifying code experiments on Nehalem, with
one CPU writing call destinations while the other CPUs are executing
them. Reliably, one of the readers goes off into the weeds within a few
seconds.

The writing was done with just text_poke(), no #BP.

I wasn't able to figure out the pattern in the addresses of the
corrupted call sites. It wasn't cache line.

That was on Nehalem. Skylake didn't crash at all.

--
Josh