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Pinned module #997

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Pinned module #997

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517d41b
rust: introduce `InPlaceModule`
wedsonaf Apr 10, 2023
14cccdb
samples: rust: add in-place initilisation sample
wedsonaf Apr 10, 2023
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11 changes: 11 additions & 0 deletions samples/rust/Kconfig
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Expand Up @@ -20,6 +20,17 @@ config SAMPLE_RUST_MINIMAL

If unsure, say N.

config SAMPLE_RUST_INPLACE
tristate "Minimal in-place"
help
This option builds the Rust minimal module with in-place
initialisation.

To compile this as a module, choose M here:
the module will be called rust_inplace.

If unsure, say N.

config SAMPLE_RUST_PRINT
tristate "Printing macros"
help
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1 change: 1 addition & 0 deletions samples/rust/Makefile
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@@ -1,6 +1,7 @@
# SPDX-License-Identifier: GPL-2.0

obj-$(CONFIG_SAMPLE_RUST_MINIMAL) += rust_minimal.o
obj-$(CONFIG_SAMPLE_RUST_INPLACE) += rust_inplace.o
obj-$(CONFIG_SAMPLE_RUST_PRINT) += rust_print.o

subdir-$(CONFIG_SAMPLE_RUST_HOSTPROGS) += hostprogs
41 changes: 41 additions & 0 deletions samples/rust/rust_inplace.rs
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@@ -0,0 +1,41 @@
// SPDX-License-Identifier: GPL-2.0

//! Rust minimal in-place sample.

use kernel::prelude::*;

module! {
type: RustInPlace,
name: "rust_inplace",
author: "Rust for Linux Contributors",
description: "Rust minimal in-place sample",
license: "GPL",
}

#[pin_data(PinnedDrop)]
struct RustInPlace {
numbers: Vec<i32>,
}
Comment on lines +15 to +18
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I think it would be beneficial to also put something in here that people can't put into non-in-place modules, e.g. a Mutex, of coruse sync would have to land first.

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I believe the main motivation for this PR is indeed to be able to use a module-wide Mutex without having to use ctor tricks?


impl kernel::InPlaceModule for RustInPlace {
type Init = impl PinInit<Self, Error>;
fn init(_module: &'static ThisModule) -> Result<Self::Init> {
pr_info!("Rust minimal sample (init)\n");
pr_info!("Am I built-in? {}\n", !cfg!(MODULE));

let mut numbers = Vec::new();
numbers.try_push(72)?;
numbers.try_push(108)?;
numbers.try_push(200)?;

Ok(try_pin_init!(Self { numbers }))
}
}

#[pinned_drop]
impl PinnedDrop for RustInPlace {
fn drop(self: Pin<&mut Self>) {
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We have a few folks suggest that instead of using a drop implementation, that we should do have deinit or exit method in kernel::Module.

We decided against it back then, which I think was the right decision.

But I think for the pinned/static case,the ergonomics of a method in the module to clean up (which takes a Pin<&mut self>) are better I think.

We should perhaps consider it.

What do you all think?

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There are a few problems with an explicit exit/deinit function:

  • people could call it like a normal function, so you cannot do things that you can do in drop (e.g. drop other things)
  • we have to blanket impl Drop for all Modules, which would prevent people from adding their own drop functionality

We could change module! to allow custom drop impls, but I feel like keeping it like this makes more sense.

Why do you think that the ergonomics of exit would be better than PinnedDrop::drop?

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With pin-ini, we have to add (PinnedDrop) to pin_data attribute of the module, then we have to add the #[pinned_drop] attribute to the PinnedDrop impl block.

These are all magic as far as I can tell with no easy way for someone who knows about regular Drop impls to find this.

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I see your point, however if you have an explicit exit function on Module, then that could be called multiple times by user code.
You could make the function unsafe, but that feels weird to me, since then you could no longer grep for unsafe to check if they use unsafe code.
You could of course mimic the #[pinned_drop] macro that makes sure nobody can call the function accidentally, but then we could also just use the drop approach.

I am not sure how we could improve the situation...

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To make it not callable from other places, we can have an extra argument of some type T whose constructor is unsafe.

Note, however, that I'm not convinced yet that an exit function in Module is necessarily the way to go. I'm just saying we should consider it.

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To make it not callable from other places, we can have an extra argument of some type T whose constructor is unsafe.

That is how #[pinned_drop] currently does things (it adds this parameter to the function). I think it would also be a source of confusion for users to have this visible.

Another thing to consider with the exit function approach is that we then need to have a blanket impl of Drop for all modules. Users will not be able to have a normal drop function, which I think is unfortunate, since that will also create confusion.

pr_info!("My numbers are {:?}\n", self.numbers);
pr_info!("Rust minimal inplace sample (exit)\n");
}
}