On 2/2/24 11:55, Alice Ryhl wrote: > The existing `CondVar` abstraction is a wrapper around > `wait_queue_head`, but it does not support all use-cases of the C > `wait_queue_head` 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_queue_head` 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_queue_head` 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_queue_head`, 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 store a > boolean next to the `wait_queue_head` to keep track of whether a > `poll_table` has ever been registered. > > Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx> > --- > rust/bindings/bindings_helper.h | 1 + > rust/kernel/sync.rs | 1 + > rust/kernel/sync/poll.rs | 117 ++++++++++++++++++++++++++++++++ > 3 files changed, 119 insertions(+) > create mode 100644 rust/kernel/sync/poll.rs Reviewed-by: Benno Lossin <benno.lossin@xxxxxxxxx> -- Cheers, Benno
