Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752090AbaDRI1m (ORCPT ); Fri, 18 Apr 2014 04:27:42 -0400 Received: from merlin.infradead.org ([205.233.59.134]:59028 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751691AbaDRI1e (ORCPT ); Fri, 18 Apr 2014 04:27:34 -0400 Date: Fri, 18 Apr 2014 10:27:16 +0200 From: Peter Zijlstra To: Waiman Long Cc: Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , linux-arch@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, xen-devel@lists.xenproject.org, kvm@vger.kernel.org, Paolo Bonzini , Konrad Rzeszutek Wilk , "Paul E. McKenney" , Rik van Riel , Linus Torvalds , Raghavendra K T , David Vrabel , Oleg Nesterov , Gleb Natapov , Scott J Norton , Chegu Vinod Subject: Re: [PATCH v9 05/19] qspinlock: Optimize for smaller NR_CPUS Message-ID: <20140418082716.GZ11096@twins.programming.kicks-ass.net> References: <1397747051-15401-1-git-send-email-Waiman.Long@hp.com> <1397747051-15401-6-git-send-email-Waiman.Long@hp.com> <20140417155649.GR11096@twins.programming.kicks-ass.net> <53504BB3.4010009@hp.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <53504BB3.4010009@hp.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 17, 2014 at 05:46:27PM -0400, Waiman Long wrote: > On 04/17/2014 11:56 AM, Peter Zijlstra wrote: > >On Thu, Apr 17, 2014 at 11:03:57AM -0400, Waiman Long wrote: > >>+struct __qspinlock { > >>+ union { > >>+ atomic_t val; char bytes[4]; > >>+ struct { > >>+#ifdef __LITTLE_ENDIAN > >>+ u16 locked_pending; > >>+ u16 tail; > >>+#else > >>+ u16 tail; > >>+ u16 locked_pending; > >>+#endif > >>+ }; struct { #ifdef __LITTLE_ENDIAN u8 locked; #else u8 res[3]; u8 locked; #endif }; > >>+ }; > >>+}; > >>+ > >>+/** > >>+ * clear_pending_set_locked - take ownership and clear the pending bit. > >>+ * @lock: Pointer to queue spinlock structure > >>+ * @val : Current value of the queue spinlock 32-bit word > >>+ * > >>+ * *,1,0 -> *,0,1 > >>+ */ > >>+static __always_inline void > >>+clear_pending_set_locked(struct qspinlock *lock, u32 val) > >>+{ > >>+ struct __qspinlock *l = (void *)lock; > >>+ > >>+ ACCESS_ONCE(l->locked_pending) = 1; > >You lost the __constant_le16_to_cpu(_Q_LOCKED_VAL) there. The > >unconditional 1 is wrong. You also have to flip the bytes in > >locked_pending. > > I don't think that is wrong. The lock byte is in the least significant 8 > bits and the pending byte is the next higher significant 8 bits irrespective > of the endian-ness. So a value of 1 in a 16-bit context means the lock byte > is set, but the pending byte is cleared. The name "locked_pending" doesn't > mean that locked variable is in a lower address than pending. val is LE bytes[0,1,2,3] BE [3,2,1,0] locked_pending is LE bytes[0,1] BE [1,0] locked LE bytes[0] BE [0] That does mean that the LSB of BE locked_pending is bytes[1]. So if you do BE: locked_pending = 1, you set bytes[1], not bytes[0]. -- 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/