Re: [RFC] LKMM: Add volatile_if()

Peter Zijlstra
2021-06-04T13:56:35Z

On Fri, Jun 04, 2021 at 02:44:22PM +0100, Will Deacon wrote:
> On Fri, Jun 04, 2021 at 01:31:48PM +0200, Peter Zijlstra wrote:
> > On Fri, Jun 04, 2021 at 11:44:00AM +0100, Will Deacon wrote:
> > > On Fri, Jun 04, 2021 at 12:12:07PM +0200, Peter Zijlstra wrote:
> >
> > > > Usage of volatile_if requires the @cond to be headed by a volatile load
> > > > (READ_ONCE() / atomic_read() etc..) such that the compiler is forced to
> > > > emit the load and the branch emitted will have the required
> > > > data-dependency. Furthermore, volatile_if() is a compiler barrier, which
> > > > should prohibit the compiler from lifting anything out of the selection
> > > > statement.
> > >
> > > When building with LTO on arm64, we already upgrade READ_ONCE() to an RCpc
> > > acquire. In this case, it would be really good to avoid having the dummy
> > > conditional branch somehow, but I can't see a good way to achieve that.
> >
> > #ifdef CONFIG_LTO
> > /* Because __READ_ONCE() is load-acquire */
> > #define volatile_cond(cond) (cond)
> > #else
> > ....
> > #endif
> >
> > Doesn't work? Bit naf, but I'm thinking it ought to do.
>
> The problem is with relaxed atomic RMWs; we don't upgrade those to acquire
> atm as they're written in asm, but we'd need volatile_cond() to work with
> them. It's a shame, because we only have RCsc RMWs on arm64, so it would
> be a bit more expensive.

Urgh, I see. Compiler can't really help in that case either I'm afraid.
They'll never want to modify loads that originate in an asm(). They'll
say to use the C11 _Atomic crud.