Re: [PATCH v3] rust: add global lock support

[Alice Ryhl <aliceryhl@google.com>]

On Fri, Sep 13, 2024 at 6:02 PM Simona Vetter <simona.vetter@ffwll.ch> wrote:
>
> On Tue, Sep 10, 2024 at 02:23:34PM +0000, Alice Ryhl wrote:
> > Add support for creating global variables that are wrapped in a mutex or
> > spinlock. Optionally, the macro can generate a special LockedBy type
> > that does not require a runtime check.
> >
> > The implementation here is intended to replace the global mutex
> > workaround found in the Rust Binder RFC [1]. In both cases, the global
> > lock must be initialized before first use. The macro is unsafe to use
> > for the same reason.
> >
> > The separate initialization step is required because bindgen refuses to
> > expose __ARCH_SPIN_LOCK_UNLOCKED to Rust as a compile-time constant. It
> > just generates an `extern "C"` global reference instead. In the future,
> > we could expose the value of __ARCH_SPIN_LOCK_UNLOCKED to Rust in a way
> > that Rust can understand. This would remove the need for a separate
> > initialization step.
>
> Yeah it's just a raw C struct initializer, I wouldn't even know how to
> move that to rust.
>
> An absolute horrible idea, and I didn't try whether it's even possible:
> - put all the global locks of a type into a special linker section (we
>   need a macro anyway for them).
> - patch them up with a horrible linker script objtool patching with an
>   example lock initialized by the C macro.
>
> Even worse idea, on most architectures/config it's all zeros. Iirc the one
> I've found that might matter a bit is CONFIG_SMP=n with some lock
> debugging enabled. We could make rust support conditional on those, and
> then maybe a build-time check that it's actually all set to 0 ...
>
> Requiring the unsafe init just feels a bit disappointing to me, when the C
> side (including lockdep annotations) tries really hard to make global
> locks a one-liner.

I actually have a prototype lying around that gets rid of the
initialization step. The idea is to define some new macros:

 #define __ARCH_SPIN_LOCK_UNLOCKED      { 0 }
+#define __ARCH_SPIN_LOCK_UNLOCKED_TYP  unsigned int
+#define __ARCH_SPIN_LOCK_UNLOCKED_INT  0

Rust then uses the two new #defines to initialize the raw spin lock to
the right bytes. As long as __ARCH_SPIN_LOCK_UNLOCKED and
__ARCH_SPIN_LOCK_UNLOCKED_INT are represented by the same sequence of
bytes, it should work.

Alice