From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754124Ab1HXM1Z (ORCPT ); Wed, 24 Aug 2011 08:27:25 -0400 Received: from casper.infradead.org ([85.118.1.10]:47923 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752005Ab1HXM1X convert rfc822-to-8bit (ORCPT ); Wed, 24 Aug 2011 08:27:23 -0400 Subject: [ia64] Question on __ARCH_WANT_UNLOCKED_CTXSW From: Peter Zijlstra To: Ken Chen , "Luck,Tony" Cc: linux-kernel , Ralf Baechle , David Miller Date: Wed, 24 Aug 2011 14:27:13 +0200 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT X-Mailer: Evolution 3.0.2- Message-ID: <1314188833.6925.3.camel@twins> Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Ken, Tony, happen to remember what the perceived benefit of using __ARCH_WANT_UNLOCKED_CTXSW was about? --- commit f8efa27662532ad5adb2790bfc3f4c78e019cfad Author: Chen, Kenneth W Date: Thu Jan 26 18:24:59 2006 -0800 [IA64] remove staled comments in asm/system.h With the recent optimization made to wrap_mmu_context function, we don't hold tasklist_lock anymore when wrapping context id. The comments in asm/system.h must fall through the crack earlier. Remove staled comments. I believe it is still beneficial to unlock the runqueue lock across context switch. So leave __ARCH_WANT_UNLOCKED_CTXSW on. Signed-off-by: Ken Chen Signed-off-by: Tony Luck diff --git a/include/asm-ia64/system.h b/include/asm-ia64/system.h index 80c5a23..0625387 100644 --- a/include/asm-ia64/system.h +++ b/include/asm-ia64/system.h @@ -249,32 +249,7 @@ extern void ia64_load_extra (struct task_struct *task); # define switch_to(prev,next,last) __switch_to(prev, next, last) #endif -/* - * On IA-64, we don't want to hold the runqueue's lock during the low-level context-switch, - * because that could cause a deadlock. Here is an example by Erich Focht: - * - * Example: - * CPU#0: - * schedule() - * -> spin_lock_irq(&rq->lock) - * -> context_switch() - * -> wrap_mmu_context() - * -> read_lock(&tasklist_lock) - * - * CPU#1: - * sys_wait4() or release_task() or forget_original_parent() - * -> write_lock(&tasklist_lock) - * -> do_notify_parent() - * -> wake_up_parent() - * -> try_to_wake_up() - * -> spin_lock_irq(&parent_rq->lock) - * - * If the parent's rq happens to be on CPU#0, we'll wait for the rq->lock - * of that CPU which will not be released, because there we wait for the - * tasklist_lock to become available. - */ #define __ARCH_WANT_UNLOCKED_CTXSW - #define ARCH_HAS_PREFETCH_SWITCH_STACK #define ia64_platform_is(x) (strcmp(x, platform_name) == 0)