From nobody Thu Dec 18 20:15:07 2025 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id EC779EE8019 for ; Fri, 8 Sep 2023 15:43:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S244786AbjIHPn6 (ORCPT ); Fri, 8 Sep 2023 11:43:58 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54348 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237259AbjIHPny (ORCPT ); Fri, 8 Sep 2023 11:43:54 -0400 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DA01813E; Fri, 8 Sep 2023 08:43:50 -0700 (PDT) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 87F46C433C9; Fri, 8 Sep 2023 15:43:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1694187830; bh=nvtvLaa0FoHyowq+s+FNMuvJVWJKzmwnQCxC99FMXow=; h=From:To:Cc:Subject:Date:From; b=taR3O9MNcKUhRi9Ue07COaSLrdRP7Bwc2UjsV2YdJabPs79M5xh0VL8nlBps3QU+g byQzj9RXhIGd7LS4AF0A8W7mTY6sMDL8YCg1wLN9MYueIVFiAwbI77YWVX1ZpEJDYP r/wC3TZyyer76nRla5GLZxG7KQLbvZVwkkr/bPsx7oUlldrV5JXeqsS46WIuzCtFsE csbmxrSLSWULw2SJcgPdfU035pVRDa9g1vx2DTpGh6eHn80DXo+0xm7+mMFEde8+Fv icCwd8P9HyqCBKQhEUme310+4zqQ+PMeyyhfIUb79PkpGCV1e5ppOBvtqPoizZUIz5 4JyW3XdFN4Ivg== From: guoren@kernel.org To: guoren@kernel.org, David.Laight@ACULAB.COM, will@kernel.org, peterz@infradead.org, mingo@redhat.com, longman@redhat.com, maobibo@loongson.cn, mjguzik@gmail.com Cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org, Guo Ren Subject: [PATCH V3] asm-generic: ticket-lock: Optimize arch_spin_value_unlocked Date: Fri, 8 Sep 2023 11:43:39 -0400 Message-Id: <20230908154339.3250567-1-guoren@kernel.org> X-Mailer: git-send-email 2.36.1 MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" From: Guo Ren The arch_spin_value_unlocked of ticket-lock would cause the compiler to generate inefficient asm code in riscv architecture because of unnecessary memory access to the contended value. Before the patch: Acked-by: Waiman Long Acked-by: Will Deacon ------ void lockref_get(struct lockref *lockref) { 78: fd010113 add sp,sp,-48 7c: 02813023 sd s0,32(sp) 80: 02113423 sd ra,40(sp) 84: 03010413 add s0,sp,48 0000000000000088 <.LBB296>: CMPXCHG_LOOP( 88: 00053783 ld a5,0(a0) ------ After the patch: ------ void lockref_get(struct lockref *lockref) { CMPXCHG_LOOP( 78: 00053783 ld a5,0(a0) ------ After the patch, the lockref_get could get in a fast path instead of the function's prologue. This is because ticket lock complex logic would limit compiler optimization for the spinlock fast path, and qspinlock won't. The caller of arch_spin_value_unlocked() could benefit from this change. Currently, the only caller is lockref. Acked-by: Waiman Long Acked-by: Will Deacon Signed-off-by: Guo Ren Signed-off-by: Guo Ren --- Changelog V3: - Add Acked-by tags - Optimize commit log V2: - Fixup commit log with Waiman advice. - Add Waiman comment in the commit msg. --- include/asm-generic/spinlock.h | 16 +++++++++------- 1 file changed, 9 insertions(+), 7 deletions(-) diff --git a/include/asm-generic/spinlock.h b/include/asm-generic/spinlock.h index fdfebcb050f4..90803a826ba0 100644 --- a/include/asm-generic/spinlock.h +++ b/include/asm-generic/spinlock.h @@ -68,11 +68,18 @@ static __always_inline void arch_spin_unlock(arch_spinl= ock_t *lock) smp_store_release(ptr, (u16)val + 1); } =20 +static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock) +{ + u32 val =3D lock.counter; + + return ((val >> 16) =3D=3D (val & 0xffff)); +} + static __always_inline int arch_spin_is_locked(arch_spinlock_t *lock) { - u32 val =3D atomic_read(lock); + arch_spinlock_t val =3D READ_ONCE(*lock); =20 - return ((val >> 16) !=3D (val & 0xffff)); + return !arch_spin_value_unlocked(val); } =20 static __always_inline int arch_spin_is_contended(arch_spinlock_t *lock) @@ -82,11 +89,6 @@ static __always_inline int arch_spin_is_contended(arch_s= pinlock_t *lock) return (s16)((val >> 16) - (val & 0xffff)) > 1; } =20 -static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock) -{ - return !arch_spin_is_locked(&lock); -} - #include =20 #endif /* __ASM_GENERIC_SPINLOCK_H */ --=20 2.36.1