Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933429AbbDITnz (ORCPT ); Thu, 9 Apr 2015 15:43:55 -0400 Received: from g4t3425.houston.hp.com ([15.201.208.53]:50285 "EHLO g4t3425.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933249AbbDITnw (ORCPT ); Thu, 9 Apr 2015 15:43:52 -0400 Message-ID: <1428608618.12911.9.camel@j-VirtualBox> Subject: Re: [PATCH 2/2] locking/rwsem: Use a return variable in rwsem_spin_on_owner() From: Jason Low To: Linus Torvalds Cc: Paul McKenney , Ingo Molnar , Peter Zijlstra , Davidlohr Bueso , Tim Chen , Aswin Chandramouleeswaran , LKML , jason.low2@hp.com Date: Thu, 09 Apr 2015 12:43:38 -0700 In-Reply-To: References: <1428521960-5268-1-git-send-email-jason.low2@hp.com> <1428521960-5268-3-git-send-email-jason.low2@hp.com> <20150409053725.GB13871@gmail.com> <1428561611.3506.78.camel@j-VirtualBox> <20150409075311.GA4645@gmail.com> <20150409175652.GI6464@linux.vnet.ibm.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.3-0ubuntu6 Content-Transfer-Encoding: 7bit Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1966 Lines: 53 On Thu, 2015-04-09 at 11:16 -0700, Linus Torvalds wrote: > On Thu, Apr 9, 2015 at 11:08 AM, Linus Torvalds > wrote: > > > > The pointer is a known-safe kernel pointer - it's just that it was > > "known safe" a few instructions ago, and might be rcu-free'd at any > > time. > > Actually, we could even do something like this: > > static inline int sem_owner_on_cpu(struct semaphore *sem, struct > task_struct *owner) > { > int on_cpu; > > #ifdef CONFIG_DEBUG_PAGEALLOC > rcu_read_lock(); > #endif > on_cpu = sem->owner == owner && owner->on_cpu; > #ifdef CONFIG_DEBUG_PAGEALLOC > rcu_read_unlock(); > #endif > return on_cpu; > } > > because we really don't need to hold the RCU lock over the whole loop, > we just need to validate that the semaphore owner still matches, and > if so, check that it's on_cpu. > > And if CONFIG_DEBUG_PAGEALLOC is set, we don't care about performance > *at*all*. We will have worse performance problems than doing some RCU > read-locking inside the loop. > > And if CONFIG_DEBUG_PAGEALLOC isn't set, we don't really care about > locking, since at worst we just access stale memory for one iteration. > > Hmm. It's not pretty, but neither is the current "let's just take a > rcu lock that we don't really need over a loop that doesn't have very > strict bounding". > > Comments? So that looks more similar to how the original code was where the rcu_read_lock() and rcu_read_unlock() was done inside the owner_running helper function (though without the CONFIG_DEBUG_PAGEALLOC), before commit 307bf9803f25 ("sched: Simplify mutex_spin_on_owner()") modified it to be done outside the loop. -- 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/