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(p5342157-ipxg23901hodogaya.kanagawa.ocn.ne.jp. [180.39.242.157]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-c0c2bfa0845sm3247728a12.28.2025.12.11.15.09.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 11 Dec 2025 15:09:44 -0800 (PST) From: FUJITA Tomonori To: ojeda@kernel.org, a.hindborg@kernel.org Cc: aliceryhl@google.com, bjorn3_gh@protonmail.com, boqun.feng@gmail.com, dakr@kernel.org, gary@garyguo.net, lossin@kernel.org, tmgross@umich.edu, rust-for-linux@vger.kernel.org Subject: [PATCH v1 1/3] rust: sync: set_once: Add fast-path check to avoid expensive cmpxchg Date: Fri, 12 Dec 2025 08:09:17 +0900 Message-ID: <20251211230919.1303926-1-fujita.tomonori@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Check if already initialized before calling cmpxchg in populate(). When SetOnce is already populated, this avoids the expensive LOCK CMPXCHG operation and uses a simple load instead. The Relaxed load is safe here because it's only a hint; the subsequent cmpxchg provides the actual synchronization. Even if we read a stale value (0), the cmpxchg will correctly fail. Signed-off-by: FUJITA Tomonori --- rust/kernel/sync/set_once.rs | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/rust/kernel/sync/set_once.rs b/rust/kernel/sync/set_once.rs index bdba601807d8..889d37dfd17f 100644 --- a/rust/kernel/sync/set_once.rs +++ b/rust/kernel/sync/set_once.rs @@ -80,6 +80,10 @@ pub fn as_ref(&self) -> Option<&T> { /// /// Returns `true` if the [`SetOnce`] was successfully populated. pub fn populate(&self, value: T) -> bool { + // Avoid expensive cmpxchg if already initialized. + if self.init.load(Relaxed) != 0 { + return false; + } // INVARIANT: If the swap succeeds: // - We increase `init`. // - We write the valid value `1` to `init`. base-commit: d358e5254674b70f34c847715ca509e46eb81e6f -- 2.43.0