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Sat, 31 Oct 2026 08:09:52 -0400 (EDT) Date: Sat, 31 Oct 2026 21:09:50 +0900 (JST) Message-Id: <20261031.210950.869638156868262129.tomo@flapping.org> To: jenhaochen@google.com Cc: gregkh@linuxfoundation.org, arve@android.com, tkjos@android.com, maco@android.com, joelagnelf@nvidia.com, brauner@kernel.org, cmllamas@google.com, aliceryhl@google.com, ojeda@kernel.org, boqun@kernel.org, gary@garyguo.net, bjorn3_gh@protonmail.com, lossin@kernel.org, a.hindborg@kernel.org, tmgross@umich.edu, dakr@kernel.org, fujita.tomonori@gmail.com, qyousef@google.com, benwei@google.com, rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] rust_binder: use Atomic::add() in BinderStats From: FUJITA Tomonori In-Reply-To: <20261031030801.4100693-1-jenhaochen@google.com> References: <20261031030801.4100693-1-jenhaochen@google.com> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit On Sat, 31 Oct 2026 03:08:01 +0000 Jenhao Chen wrote: > BinderStats::inc_bc() and BinderStats::inc_br() increment command and > return statistics counters on every Binder transaction and reply using > Atomic::fetch_add(1, Relaxed), discarding the returned previous value. > > Because the return value of fetch_add() is unused in Rust but must still > be produced by the underlying value-returning atomic RMW helper > (atomic_fetch_add_relaxed()), the compiler is forced to emit a > value-returning atomic operation (LDADD on arm64 with LSE, or LOCK XADD > on x86) instead of a non-value-returning atomic add (STADD on arm64, or > LOCK ADD on x86). On arm64 CPUs with LSE, value-returning LDADD requires > a round-trip response from the L2/interconnect to populate the > destination register, whereas non-value-returning STADD (encoded as > LDADD with WZR as the destination register) may be executed as a far > atomic without waiting for the old value to return to the core. > > Switch BinderStats::inc_bc() and BinderStats::inc_br() from > fetch_add(1, Relaxed) to add(1, Relaxed) so they lower to atomic_add() > and emit non-value-returning atomic increments. This also matches the C > Binder driver, which uses atomic_inc() for these statistics counters in > binder_thread_write() and binder_stat_br(). > > No functional change intended. > > Signed-off-by: Jenhao Chen Reviewed-by: FUJITA Tomonori