Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752165Ab3FXVak (ORCPT ); Mon, 24 Jun 2013 17:30:40 -0400 Received: from mga09.intel.com ([134.134.136.24]:35526 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752117Ab3FXVag (ORCPT ); Mon, 24 Jun 2013 17:30:36 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.87,931,1363158000"; d="scan'208";a="334861716" Subject: Re: [PATCH 2/2] rwsem: do optimistic spinning for writer lock acquisition From: Tim Chen To: Peter Hurley Cc: Davidlohr Bueso , Alex Shi , Michel Lespinasse , Ingo Molnar , Andrew Morton , Andrea Arcangeli , Andi Kleen , Matthew R Wilcox , Dave Hansen , Peter Zijlstra , Rik van Riel , linux-kernel@vger.kernel.org, linux-mm In-Reply-To: <51C8B0AA.4070204@hurleysoftware.com> References: <1371858700.22432.5.camel@schen9-DESK> <51C558E2.1040108@hurleysoftware.com> <1372017836.1797.14.camel@buesod1.americas.hpqcorp.net> <1372093876.22432.34.camel@schen9-DESK> <51C894C3.4040407@hurleysoftware.com> <1372105065.22432.65.camel@schen9-DESK> <51C8B0AA.4070204@hurleysoftware.com> Content-Type: text/plain; charset="UTF-8" Date: Mon, 24 Jun 2013 14:30:38 -0700 Message-ID: <1372109438.22432.75.camel@schen9-DESK> Mime-Version: 1.0 X-Mailer: Evolution 2.32.3 (2.32.3-1.fc14) Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 978 Lines: 27 On Mon, 2013-06-24 at 16:48 -0400, Peter Hurley wrote: > > Also, I haven't given a lot of thought to if preemption must be disabled > before calling rwsem_can_spin_on_owner(). If so, wouldn't you just drop > rwsem_can_spin_on_owner() (because the conditions tested in the loop are > equivalent)? > Not totally equivalent. If we drop the call to rwsem_can_spin_on_owner, we will spin when readers are holding the lock (owner is null). Right now we only allow writers to spin when other writers are holding the lock by adding the rwsem_can_spin_on_owner check. Letting spinning on readers held lock is tricky as we could have a reader that sleeps and if we don't detect the case. We could spin for too long. Thanks. Tim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/