Re: [PATCH v8 6/7] apei/ghes: Use unrcu_pointer for cmpxchg

Peter Zijlstra
2022-10-13T16:46:17Z

On Thu, Oct 13, 2022 at 05:41:06PM +0200, Ard Biesheuvel wrote:
> On Thu, 13 Oct 2022 at 15:37, Borislav Petkov <bp@alien8.de> wrote:
> >
> > On Wed, Oct 12, 2022 at 12:04:57PM +0000, Justin He wrote:
> > > I have a concern about what if cmpxchg failed? Do we have to still
> > > guarantee the ordering since cmpxchg will not imply a smp_mb if it
> > > failed.
> >
> > Of course it will imply that. At least on x86 it does. smp_wmb() is a
> > compiler barrier there and cmpxchg() already has that barrier semantics
> > by clobbering "memory". I'm pretty sure you should have the same thing
> > on ARM.
> >
>
> No it definitely does not imply that. A memory clobber is a codegen
> construct, and the hardware could still complete the writes in a way
> that could result in another observer seeing a mix of old and new
> values that is inconsistent with the ordering of the stores as issued
> by the compiler.

Borislav is thinking too much x86. Failed cmpxchg() does indeed not
imply any memory ordering for all architectures -- and while the memory
clobber (aka. barrier()) is equivalent to an smp_wmb() on x86, that most
certainly doesn't hold for non x86 code.

> > says, "new_cache must be put into array after its contents are written".
> >
> > Are we writing anything into the cache if cmpxchg fails?
> >
>
> The cache fields get updated but the pointer to the struct is never
> shared globally if the cmpxchg() fails so not having the barrier on
> failure should not be an issue here.

That is how I read the code too; so if the cmpxchg fails the object is
not published and nobody cares about the ordering.

>
> > > Besides, I didn't find the paired smp_mb or smp_rmb for this smp_wmb.
> >
> > Why would there be pairs? I don't understand that statement here.
> >
>
> Typically, the other observer pairs the write barrier with a read barrier.
>
> In this case, the other observer appears to be ghes_estatus_cached(),
> and the reads of the cache struct fields must be ordered after the
> read of the cache struct's address. However, there is an implicit
> ordering there through an address dependency (you cannot dereference a
> struct without knowing its address) so the ordering is implied (and
> rcu_dereference() has a READ_ONCE() inside so we are guaranteed to
> always dereference the same struct, even if the array slot gets
> updated concurrently.)
>
> If you want to get rid of the barrier, you could drop it and change
> the cmpxchg() to cmpxchg_release().

cmpxchg_release() is strictly weaker than cmpxchg(); so I don't see the
point there other than optimizing for weak architectures. It can't
fundamentally fix anything.