linux-kernel - [PATCH RFC 0/2] Avoid synchronize_rcu() for every thread drop in Rust Binder

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Message-ID: <20260117-upgrade-poll-v1-0-179437b7bd49@google.com>
Date: Sat, 17 Jan 2026 15:25:18 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Christian Brauner <brauner@...nel.org>, Boqun Feng <boqun.feng@...il.com>, 
	"Paul E. McKenney" <paulmck@...nel.org>
Cc: Gary Guo <gary@...yguo.net>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Carlos Llamas <cmllamas@...gle.com>, linux-fsdevel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Alice Ryhl <aliceryhl@...gle.com>
Subject: [PATCH RFC 0/2] Avoid synchronize_rcu() for every thread drop in Rust Binder

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.

One solution that may be better is to just kfree_rcu() the Binder Thread
struct directly when it's refcount drops to zero (Drop of everything
else can still run without a grace period). But I'm not sure how I would
achieve that - after all Thread is also used with kernel::list.

Based on top of:
https://lore.kernel.org/all/20260117122243.24404-5-boqun.feng@gmail.com/

Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
---
Alice Ryhl (2):
      rust: poll: make PollCondVar upgradable
      rust_binder: use UpgradePollCondVar

 drivers/android/binder/process.rs |   2 +-
 drivers/android/binder/thread.rs  |  18 +++--
 rust/kernel/sync/poll.rs          | 163 +++++++++++++++++++++++++++++++++++++-
 3 files changed, 173 insertions(+), 10 deletions(-)
---
base-commit: e0a15d3fd5100c25d1e06837bdb3070a7a716e32
change-id: 20260117-upgrade-poll-37ee2a7a79dd

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