Re: [PATCH v4 4/4] arm64: lto: Strengthen READ_ONCE() to acquire when CONFIG_LTO=y

[Will Deacon]

On Tue, Nov 03, 2020 at 12:58:45PM +0000, Mark Rutland wrote:
> On Tue, Nov 03, 2020 at 12:17:21PM +0000, Will Deacon wrote:
> > When building with LTO, there is an increased risk of the compiler
> > converting an address dependency headed by a READ_ONCE() invocation
> > into a control dependency and consequently allowing for harmful
> > reordering by the CPU.
> > 
> > Ensure that such transformations are harmless by overriding the generic
> > READ_ONCE() definition with one that provides acquire semantics when
> > building with LTO.
> > 
> > Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > Signed-off-by: Will Deacon <will@xxxxxxxxxx>
> 
> [...]
> 
> Could we add a note above __READ_ONCE() along the lines of the commit
> message, e.g.
> 
> /*
>  * With LTO a compiler might convert an address dependency headed by a
>  * READ_ONCE() into a control dependency, allowing for harmful
>  * reordering by the CPU.
>  *
>  * To prevent this, upgrade READ_OONCE() to provide acquire semantics
>  * when building with LTO.

It's not halloween any moooore :)

But yes, I'll add something to that effect, cheers.

Will