Re: C aggregate passing (Rust kernel policy)

Kent Overstreet
2025-02-28T15:41:22Z

On Fri, Feb 28, 2025 at 08:44:58AM +0100, Ralf Jung wrote:
> Hi,
>
> > > I guess you can sum this up to:
> > >
> > > The compiler should never assume it's safe to read a global more than the
> > > code specifies, but if the code reads a global more than once, it's fine
> > > to cache the multiple reads.
> > >
> > > Same for writes, but I find WRITE_ONCE() used less often than READ_ONCE().
> > > And when I do use it, it is more to prevent write tearing as you mentioned.
> >
> > Except that (IIRC) it is actually valid for the compiler to write something
> > entirely unrelated to a memory location before writing the expected value.
> > (eg use it instead of stack for a register spill+reload.)
> > Not gcc doesn't do that - but the standard lets it do it.
>
> Whether the compiler is permitted to do that depends heavily on what exactly
> the code looks like, so it's hard to discuss this in the abstract.
> If inside some function, *all* writes to a given location are atomic (I
> think that's what you call WRITE_ONCE?), then the compiler is *not* allowed
> to invent any new writes to that memory. The compiler has to assume that
> there might be concurrent reads from other threads, whose behavior could
> change from the extra compiler-introduced writes. The spec (in C, C++, and
> Rust) already works like that.
>
> OTOH, the moment you do a single non-atomic write (i.e., a regular "*ptr =
> val;" or memcpy or so), that is a signal to the compiler that there cannot
> be any concurrent accesses happening at the moment, and therefore it can
> (and likely will) introduce extra writes to that memory.

Is that how it really works?

I'd expect the atomic writes to have what we call "compiler barriers"
before and after; IOW, the compiler can do whatever it wants with non
atomic writes, provided it doesn't cross those barriers.