Re: [PATCH] VMCI: Fixup atomic64_t abuse

From: Peter Zijlstra
Date: Thu Jun 06 2019 - 13:08:21 EST

On Thu, Jun 06, 2019 at 03:54:24PM +0000, Jorgen Hansen wrote:
> > On 6 Jun 2019, at 11:34, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> >
> > The VMCI driver is abusing atomic64_t and atomic_t, there is no actual
> > atomic RmW operations around.
> >
> > Rewrite the code to use a regular u64 with READ_ONCE() and
> > WRITE_ONCE() and a cast to 'unsigned long'. This fully preserves
> > whatever broken there was (it's not endian-safe for starters, and also
> > looks to be missing ordering).
> Thanks for the cleanup.
> This code is only intended for use with the vmci device driver, and
> that is X86 only, so during the original upstreaming no effort was
> made to make this work correctly on anything else.

Even x86 needs compiler barriers to ensure the compiler emits the
instructions in the expected order.

> Weâll be updating the vmci device driver to work on other
> architectures soonish, so will be adding barriers to enforce ordering
> as well at that point. If you want to leave your patch as is, we can
> address the type casting then.

You might want to have a look at smp_store_release() and
smp_load_acquire() I suppose.