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

[David Laight]

From: Josh Poimboeuf
> Sent: 30 November 2018 16:27
> 
> 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@xxxxxxxxxx> wrote:
...
> > > 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.

Interesting thought?

If it is possible to add a prefix that can be overwritten by an int3
is it also possible to add something that the assembler will use
to align the instruction so that a write to the 4 byte offset
will be atomic?

I'd guess that avoiding 8 byte granularity would be sufficient.
So you'd need a 1, 2 or 3 byte nop depending on the actual
alignment - although a 3 byte one would always do.

	David

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