Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756046AbbDISjg (ORCPT ); Thu, 9 Apr 2015 14:39:36 -0400 Received: from e39.co.us.ibm.com ([32.97.110.160]:52260 "EHLO e39.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755680AbbDISjd (ORCPT ); Thu, 9 Apr 2015 14:39:33 -0400 Date: Thu, 9 Apr 2015 11:39:26 -0700 From: "Paul E. McKenney" To: Linus Torvalds Cc: Ingo Molnar , Jason Low , Peter Zijlstra , Davidlohr Bueso , Tim Chen , Aswin Chandramouleeswaran , LKML Subject: Re: [PATCH 2/2] locking/rwsem: Use a return variable in rwsem_spin_on_owner() Message-ID: <20150409183926.GM6464@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15040918-0033-0000-0000-0000043012EB Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2048 Lines: 59 On Thu, Apr 09, 2015 at 11:16:24AM -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. Much better from an RCU grace-period-latency perspective. > 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. But if we are running on a hypervisor, mightn't our VCPU be preempted just before accessing ->on_cpu, the task exit and its structures be freed and unmapped? Or is the task structure in memory that is never unmapped? (If the latter, clearly not a problem.) Thanx, Paul > 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? > > Linus > -- 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/