rust: sync: rcu: Add RCU protected pointer
RCU protected pointers are an atomic pointer that can be loaded and
dereferenced by mulitple RCU readers, but only one updater/writer can
change the value (following a read-copy-update pattern usually).
This is useful in the case where data is read-mostly. The rationale of
this patch is to provide a proof of concept on how RCU should be exposed
to the Rust world, and it also serves as an example for atomic usage.
Similar mechanisms like ArcSwap [1] are already widely used.
Provide a `Rcu<P>` type with an atomic pointer implementation. `P` has
to be a `ForeignOwnable`, which means the ownership of a object can be
represented by a pointer-size value.
`Rcu::dereference()` requires a RCU Guard, which means dereferencing is
only valid under RCU read lock protection.
`Rcu::copy_update()` is the operation for updaters, it requries a
`Pin<&mut Self>` for exclusive accesses, since RCU updaters are normally
exclusive with each other.
A lot of RCU functionalities including asynchronously free (call_rcu()
and kfree_rcu()) are still missing, and will be the future work.
Also, we still need language changes like field projection [2] to
provide better ergonomic.
Acknowledgment: this work is based on a lot of productive discussions
and hard work from others, these are the ones I can remember (sorry if I
forgot your contribution):
* Wedson started the work on RCU field projection and Benno followed it
up and had been working on it as a more general language feature.
Also, Gary's field-projection repo [3] has been used as an example for
related discussions.
* During Kangrejos 2023 [4], Gary, Benno and Alice provided a lot of
feedbacks on the talk from Paul and me: "If you want to use RCU in
Rust for Linux kernel..."
* During a recent discussion among Benno, Paul and me, Benno suggested
using `Pin<&mut>` to guarantee the exclusive access on updater
operations.
[boqun: Apply style suggestion from Dirk Behme]
Link: https://crates.io/crates/arc-swap [1]
Link: https://rust-lang.zulipchat.com/#narrow/channel/213817-t-lang/topic/Field.20Projections/near/474648059 [2]
Link: https://github.com/nbdd0121/field-projection [3]
Link: https://kangrejos.com/2023 [4]
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
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