Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1945926AbaD3RuN (ORCPT ); Wed, 30 Apr 2014 13:50:13 -0400 Received: from mga09.intel.com ([134.134.136.24]:1794 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758905AbaD3RuL (ORCPT ); Wed, 30 Apr 2014 13:50:11 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.97,959,1389772800"; d="scan'208";a="503855700" Subject: Re: [PATCH v2] rwsem: Support optimistic spinning From: Tim Chen To: Peter Zijlstra Cc: Davidlohr Bueso , Ingo Molnar , Andrew Morton , Linus Torvalds , Andrea Arcangeli , Alex Shi , Andi Kleen , Michel Lespinasse , Rik van Riel , Peter Hurley , Thomas Gleixner , "Paul E.McKenney" , Aswin Chandramouleeswaran , "Norton, Scott J" , linux-kernel@vger.kernel.org In-Reply-To: <20140430082715.GA11096@twins.programming.kicks-ass.net> References: <1398205166.6345.7.camel@buesod1.americas.hpqcorp.net> <1398722941.25549.16.camel@buesod1.americas.hpqcorp.net> <20140430082715.GA11096@twins.programming.kicks-ass.net> Content-Type: text/plain; charset="UTF-8" Date: Wed, 30 Apr 2014 10:50:09 -0700 Message-ID: <1398880209.2970.100.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 On Wed, 2014-04-30 at 10:27 +0200, Peter Zijlstra wrote: > On Mon, Apr 28, 2014 at 03:09:01PM -0700, Davidlohr Bueso wrote: > > +/* > > + * Try to acquire write lock before the writer has been put on wait queue. > > + */ > > +static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem) > > +{ > > + long count = ACCESS_ONCE(sem->count); > > +retry: > > + if (count == RWSEM_WAITING_BIAS) { > > + count = cmpxchg(&sem->count, RWSEM_WAITING_BIAS, > > + RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS); > > + /* allow write lock stealing, try acquiring the write lock. */ > > + if (count == RWSEM_WAITING_BIAS) > > + goto acquired; > > + else if (count == 0) > > + goto retry; > > + } else if (count == 0) { > > + count = cmpxchg(&sem->count, 0, RWSEM_ACTIVE_WRITE_BIAS); > > + if (count == 0) > > + goto acquired; > > + else if (count == RWSEM_WAITING_BIAS) > > + goto retry; > > + } > > + return false; > > + > > +acquired: > > + return true; > > +} > > Could we have written that like: > > static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem) > { > long old, count = ACCESS_ONCE(sem->count); > > for (;;) { > if (!(count == 0 || count == RWSEM_WAITING_BIAS)) > return false; > > old = cmpxchg(&sem->count, count, count + RWSEM_ACTIVE_BIAS); Above line won't be correct for the case when count == 0. We are trying to acquire write lock, so the sem->count should become RWSEM_ACTIVE_WRITE_BIAS, or RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS. So we should change the logic to if (count == RWSEM_WAITING_BIAS) old = cmpxchg(&sem->count, count, count + RWSEM_ACTIVE_BIAS); else old = cmpxchg(&sem->count, count, count + RWSEM_ACTIVE_WRITE_BIAS); > if (old == count) > return true; > > count = old; > } > } > > ? > > Just checking my brain got all the branches untangled right :-) 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/