On Fri, Apr 16, 2021 at 4:19 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> Does this also not prohibit constructs where modification must be done
> while holding two locks, but reading can be done while holding either
> lock?
Yeah, this came up in some discussions we had. There are some locking
patterns that we need to think about how to model best within Rust's
type system.
But even if some patterns cannot be made safe, that is fine and does
not diminish the advantages everywhere else.
> I would much rather have a language extention where we can associate
> custom assertions with variable access, sorta like a sanitizer:
>
> static inline void assert_foo_bar(struct foo *f)
> {
> lockdep_assert_held(&f->lock);
> }
>
> struct foo {
> spinlock_t lock;
> int bar __assert__(assert_foo_bar);
> };
>
> Such things can be optional and only enabled for debug builds on new
> compilers.
More sanitizers and ways to check "unsafe" code is sound are always
welcome -- not just for C, also for Rust `unsafe` code (e.g. Miri).
However, the main advantage of Rust for us is its safe subset (which
handles quite a lot of patterns, thanks to the lifetime tracking /
borrow checker).
Of course, we could propose something similar for C -- in fact, there
was a recent discussion around this in the C committee triggered by my
n2659 "Safety attributes for C" paper. However, achieving that would
require a lot of work, time, new syntax, etc. It is not something that
is in the radar just yet.
Similarly, if some compiler ends up implementing an extension that
actually realizes the same guarantees as Rust, we would likely end up
wrapping everything with macros like in the guards example you
mentioned, and even then we would not have got the rest of the
advantages that Rust brings to the table.
> C does indeed not have the concept of ownership, unlike modern C++ I
> think. But I would much rather see a C language extention for that than
> go Rust.
Many "resource-like" C++ types model ownership, yes; e.g.
`std::unique_ptr` for memory, as well as a myriad of ones in different
projects for different kinds of resources, plus generic ones like the
proposed P0052. However, they do not enforce their usage is correct.
Cheers,
Miguel
