Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751258AbdHXFr3 (ORCPT ); Thu, 24 Aug 2017 01:47:29 -0400 Received: from mail-pg0-f65.google.com ([74.125.83.65]:35687 "EHLO mail-pg0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751019AbdHXFr1 (ORCPT ); Thu, 24 Aug 2017 01:47:27 -0400 From: Hoeun Ryu To: Kees Cook , Andrew Morton , Nicholas Piggin , Ingo Molnar , Michal Marek , Laura Abbott , Al Viro , Andy Lutomirski , mic@digikod.net, Russell King , Vladimir Murzin , Heiko Carstens , Jonathan Austin , Masahiro Yamada , Mark Rutland , Peter Zijlstra Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, hoeun.ryu@gmail.com, linux-arch@vger.kernel.org Subject: [RFC 0/3] add 64BIT_ATOMIC_ACCESS and 64BIT_ATOMIC_ALIGNED_ACCESS Date: Thu, 24 Aug 2017 14:42:54 +0900 Message-Id: <1503553377-3646-1-git-send-email-hoeun.ryu@gmail.com> X-Mailer: git-send-email 2.7.4 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2048 Lines: 47 On some 32-bit architectures, 64bit accesses are atomic when certain conditions are satisfied. For example, on LPAE (Large Physical Address Extension) enabled ARM architecture, 'ldrd/strd' (load/store doublewords) instructions are 64bit atomic as long as the address is 64-bit aligned. This feature is to guarantee atomic accesses on newly introduced 64bit wide descriptors in the translation tables, and 's/u64' variables can be accessed atomically when they are aligned(8) on LPAE enabled ARM architecture machines. Introducing 64BIT_ATOMIC_ACCESS and 64BIT_ATOMIC_ALIGNED_ACCESS, which can be true for the 32bit architectures as well as 64bit architectures. we can optimize some kernel codes using seqlock (timekeeping) or mimics of it (like in sched/cfq) simply to read or write 64bit variables. The existing codes depend on CONFIG_64BIT to determine whether the 64bit variables can be directly accessed or need additional synchronization primitives like seqlock. CONFIG_64BIT_ATOMIC_ACCESS can be used instead of CONFIG_64BIT in the cases. 64BIT_ATOMIC_ALIGNED_ACCESS can be used in the variable declaration to indicate the alignment requirement to the compiler (__attribute__((aligned(8)))) in the way of #ifdef. The last patch "sched: depend on 64BIT_ATOMIC_ACCESS to determine if to use min_vruntime_copy" is an example of this approach. I'd like to know what the architecture maintainers and kernel maintainers think about it. I think I can make more examples (mostly removing seqlock to access the 64bit variables on the machines) if this approach is accepted. Hoeun Ryu (3): arch: add 64BIT_ATOMIC_ACCESS to support 64bit atomic access on 32bit machines arm: enable 64BIT_ATOMIC(_ALIGNED)_ACCESS on LPAE enabled machines sched: depend on 64BIT_ATOMIC_ACCESS to determine if to use min_vruntime_copy arch/Kconfig | 20 ++++++++++++++++++++ arch/arm/mm/Kconfig | 2 ++ kernel/sched/fair.c | 6 +++--- kernel/sched/sched.h | 6 +++++- 4 files changed, 30 insertions(+), 4 deletions(-) -- 2.7.4