On Thu, Oct 26, 2023 at 01:16:25PM +0200, Peter Zijlstra wrote:
> On Thu, Oct 26, 2023 at 11:36:10AM +0100, Gary Guo wrote:
>
> > There's two reasons that we are using volatile read/write as opposed to
> > relaxed atomic:
> > * Rust lacks volatile atomics at the moment. Non-volatile atomics are
> > not sufficient because the compiler is allowed (although they
> > currently don't) optimise atomics. If you have two adjacent relaxed
> > loads, they could be merged into one.
>
> Ah yes, that would be problematic, eg, if lifted out of a loop things
> could go sideways fast.
>
Maybe we can workaround this limitation by using compiler barriers, i.e.
compiler_fence(SeqCst);
load(Relaxed);
compiler_fence(Acquire);
this is slightly stronger than a volatile atomic.
> > * Atomics only works for integer types determined by the platform. On
> > some 32-bit platforms you wouldn't be able to use 64-bit atomics at
> > all, and on x86 you get less optimal sequence since volatile load is
> > permitted to tear while atomic load needs to use LOCK CMPXCHG8B.
>
> We only grudgingly allowed u64 READ_ONCE() on 32bit platforms because
> the fallout was too numerous to fix. Some of them are probably bugs.
>
> Also, I think cmpxchg8b without lock prefix would be sufficient, but
> I've got too much of a head-ache to be sure. Worse is that we still
> support targets without cmpxchg8b.
>
> It might be interesting to make the Rust side more strict in this regard
> and see where/when we run into trouble.
>
Sounds good to me. If the compiler barriers make sense for now, then
we can do:
pub unsafe fn read_once_usize(ptr: *const usize) -> usize {
core::sync::atomic::compiler_fence(SeqCst);
let r = unsafe { *ptr.cast::<AtomicUsize>() }.load(Relaxed);
core::sync::atomic::compiler_fence(Acquire);
r
}
and if the other side (i.e. write) is also atomic (e.g. WRITE_ONCE()),
we don't have data race.
However, there are still cases where data races are ignored in C code,
for example inode::i_state: reads out of locks race with writes inside
locks, since writes are done by plain accesses. Nothing can be done to
fix that from Rust side only, and fixing the C side is a separate topic.
Thoughts?
Regards,
Boqun
> > * Atomics doesn't work for complex structs. Although I am not quite sure
> > of the value of supporting it.
>
> So on the C side we mandate the size is no larger than machine word,
> with the exception of the u64 on 32bit thing. We don't mandate strict
> integer types because things like pte_t are wrapper types.
>
>
