From 5cd4b9370220e4a4a6b80a2ce86853680c904258 Mon Sep 17 00:00:00 2001 From: Alice Ryhl Date: Wed, 6 Dec 2023 11:59:52 +0000 Subject: [PATCH] rust: file: add abstraction for `poll_table` The existing `CondVar` abstraction is a wrapper around `wait_list`, but it does not support all use-cases of the C `wait_list` type. To be specific, a `CondVar` cannot be registered with a `struct poll_table`. This limitation has the advantage that you do not need to call `synchronize_rcu` when destroying a `CondVar`. However, we need the ability to register a `poll_table` with a `wait_list` in Rust Binder. To enable this, introduce a type called `PollCondVar`, which is like `CondVar` except that you can register a `poll_table`. We also introduce `PollTable`, which is a safe wrapper around `poll_table` that is intended to be used with `PollCondVar`. The destructor of `PollCondVar` unconditionally calls `synchronize_rcu` to ensure that the removal of epoll waiters has fully completed before the `wait_list` is destroyed. That said, `synchronize_rcu` is rather expensive and is not needed in all cases: If we have never registered a `poll_table` with the `wait_list`, then we don't need to call `synchronize_rcu`. (And this is a common case in Binder - not all processes use Binder with epoll.) The current implementation does not account for this, but if we find that it is necessary to improve this, a future patch could change store a boolean next to the `wait_list` to keep track of whether a `poll_table` has ever been registered. Signed-off-by: Alice Ryhl Link: https://lore.kernel.org/r/20231206-alice-file-v2-7-af617c0d9d94@google.com [ boqun: Removes unused POLLFREE definition ] --- rust/bindings/bindings_helper.h | 1 + rust/kernel/sync.rs | 1 + rust/kernel/sync/poll.rs | 103 ++++++++++++++++++++++++++++++++ 3 files changed, 105 insertions(+) create mode 100644 rust/kernel/sync/poll.rs diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h index 6b5616499b6dad..56c1471fc03cd8 100644 --- a/rust/bindings/bindings_helper.h +++ b/rust/bindings/bindings_helper.h @@ -13,6 +13,7 @@ #include #include #include +#include #include #include #include diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs index d219ee518eff15..84726f80c4069c 100644 --- a/rust/kernel/sync.rs +++ b/rust/kernel/sync.rs @@ -11,6 +11,7 @@ mod arc; mod condvar; pub mod lock; mod locked_by; +pub mod poll; pub use arc::{Arc, ArcBorrow, UniqueArc}; pub use condvar::CondVar; diff --git a/rust/kernel/sync/poll.rs b/rust/kernel/sync/poll.rs new file mode 100644 index 00000000000000..e1dded9b7b9d69 --- /dev/null +++ b/rust/kernel/sync/poll.rs @@ -0,0 +1,103 @@ +// SPDX-License-Identifier: GPL-2.0 + +//! Utilities for working with `struct poll_table`. + +use crate::{ + bindings, + file::File, + prelude::*, + sync::{CondVar, LockClassKey}, + types::Opaque, +}; +use core::ops::Deref; + +/// Creates a [`PollCondVar`] initialiser with the given name and a newly-created lock class. +#[macro_export] +macro_rules! new_poll_condvar { + ($($name:literal)?) => { + $crate::file::PollCondVar::new($crate::optional_name!($($name)?), $crate::static_lock_class!()) + }; +} + +/// Wraps the kernel's `struct poll_table`. +#[repr(transparent)] +pub struct PollTable(Opaque); + +impl PollTable { + /// Creates a reference to a [`PollTable`] from a valid pointer. + /// + /// # Safety + /// + /// The caller must ensure that for the duration of 'a, the pointer will point at a valid poll + /// table, and that it is only accessed via the returned reference. + pub unsafe fn from_ptr<'a>(ptr: *mut bindings::poll_table) -> &'a mut PollTable { + // SAFETY: The safety requirements guarantee the validity of the dereference, while the + // `PollTable` type being transparent makes the cast ok. + unsafe { &mut *ptr.cast() } + } + + fn get_qproc(&self) -> bindings::poll_queue_proc { + let ptr = self.0.get(); + // SAFETY: The `ptr` is valid because it originates from a reference, and the `_qproc` + // field is not modified concurrently with this call since we have an immutable reference. + unsafe { (*ptr)._qproc } + } + + /// Register this [`PollTable`] with the provided [`PollCondVar`], so that it can be notified + /// using the condition variable. + pub fn register_wait(&mut self, file: &File, cv: &PollCondVar) { + if let Some(qproc) = self.get_qproc() { + // SAFETY: The pointers to `self` and `file` are valid because they are references. + // + // Before the wait list is destroyed, the destructor of `PollCondVar` will clear + // everything in the wait list, so the wait list is not used after it is freed. + unsafe { qproc(file.as_ptr() as _, cv.wait_list.get(), self.0.get()) }; + } + } +} + +/// A wrapper around [`CondVar`] that makes it usable with [`PollTable`]. +/// +/// # Invariant +/// +/// If `needs_synchronize_rcu` is false, then there is nothing registered with `register_wait`. +/// +/// [`CondVar`]: crate::sync::CondVar +#[pin_data(PinnedDrop)] +pub struct PollCondVar { + #[pin] + inner: CondVar, +} + +impl PollCondVar { + /// Constructs a new condvar initialiser. + pub fn new(name: &'static CStr, key: &'static LockClassKey) -> impl PinInit { + pin_init!(Self { + inner <- CondVar::new(name, key), + }) + } +} + +// Make the `CondVar` methods callable on `PollCondVar`. +impl Deref for PollCondVar { + type Target = CondVar; + + fn deref(&self) -> &CondVar { + &self.inner + } +} + +#[pinned_drop] +impl PinnedDrop for PollCondVar { + fn drop(self: Pin<&mut Self>) { + // Clear anything registered using `register_wait`. + // + // SAFETY: The pointer points at a valid wait list. + unsafe { bindings::__wake_up_pollfree(self.inner.wait_list.get()) }; + + // Wait for epoll items to be properly removed. + // + // SAFETY: Just an FFI call. + unsafe { bindings::synchronize_rcu() }; + } +}