Re: C aggregate passing (Rust kernel policy)

Boqun Feng
2025-02-28T15:46:27Z

On Fri, Feb 28, 2025 at 10:41:12AM -0500, Kent Overstreet wrote:
> 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

If the atomic writes are relaxed, they shouldn't have "compiler
barriers" before or after, e.g. our kernel atomics don't have such
compiler barriers. And WRITE_ONCE() is basically relaxed atomic writes.

Regards,
Boqun

> atomic writes, provided it doesn't cross those barriers.