[PATCH v5 0/2] Avoid synchronize_rcu() for every thread drop in Rust Binder

[Alice Ryhl <aliceryhl@google.com>]

Right now Rust Binder calls synchronize_rcu() more often than is
necessary. Most processes do not use epoll at all, so they don't require
rcu here. Back in Kangrejos I came up with a way to avoid this. Idea is
to move the value that needs rcu to a separate allocation that's easy to
kfree_rcu(). We pay the allocation only when the proc uses epoll using
an "upgrade" strategy - most processes don't.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
Changes in v5:
- Rebase on top of series converting node_refs to spinlock.
- Reword comment claiming node_refs is a mutex.
- Link to v4: https://lore.kernel.org/r/20260523-upgrade-poll-v4-0-f5b4c747eac2@google.com

Changes in v4:
- Use SetOnce for PollCondVar variable instead of storing inside
  spinlock. This avoids calling poll_wait() under the spinlock.
- Link to v3: https://lore.kernel.org/r/20260508-upgrade-poll-v3-0-0c619fe846e8@google.com

Changes in v3:
- This series was almost entirely rewritten to use a different
  implementation strategy. By moving the PollCondVar to the process we
  can avoid the upgrade logic entirely.
- Link to v2: https://lore.kernel.org/r/20260213-upgrade-poll-v2-0-984a0fb184fb@google.com

Changes in v2:
- Change how Rust Binder handles the lock class key.
- Rebase.
- Link to v1: https://lore.kernel.org/r/20260117-upgrade-poll-v1-0-179437b7bd49@google.com

---
Alice Ryhl (2):
      rust: poll: use kfree_rcu() for PollCondVar
      rust_binder: move (e)poll wait queue to Process

 drivers/android/binder/node.rs        |  4 +-
 drivers/android/binder/process.rs     | 67 ++++++++++++++++++++++--------
 drivers/android/binder/thread.rs      | 78 +++++++++++++++++------------------
 drivers/android/binder/transaction.rs |  6 ++-
 rust/kernel/sync/poll.rs              | 73 +++++++++++++++++++++++++++++++-
 5 files changed, 167 insertions(+), 61 deletions(-)
---
base-commit: da61573f783897ae5a96c8f1c71aad6242344feb
change-id: 20260117-upgrade-poll-37ee2a7a79dd
prerequisite-change-id: 20260608-binder-noderefs-spin-3a0ec0589043:v2
prerequisite-patch-id: 3a1c4f545b2281e2e91ea0af7fe6c71f5ae0c08e
prerequisite-patch-id: e1ae5f73b329080c0fcea1d2509c085e105f67c3
prerequisite-patch-id: 63f2092c6fd9dd54b7adf93be8d5670d8490f401
prerequisite-patch-id: 5811aeb4ca435a1d4d0bb347d4de9f3b2d91f814
prerequisite-patch-id: 556e45c697f9888c8446245bbcf478422991fddf
prerequisite-patch-id: f567ba2263108b12871013309ae7c482d500eeae

Best regards,
-- 
Alice Ryhl <aliceryhl@google.com>