Re: [PATCH] rust_binder: use Atomic::add() in BinderStats

[FUJITA Tomonori]

On Sat, 31 Oct 2026 03:08:01 +0000
Jenhao Chen <jenhaochen@xxxxxxxxxx> 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 <jenhaochen@xxxxxxxxxx>

Reviewed-by: FUJITA Tomonori <fujita.tomonori@xxxxxxxxx>