Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933507AbbDIT7B (ORCPT ); Thu, 9 Apr 2015 15:59:01 -0400 Received: from e36.co.us.ibm.com ([32.97.110.154]:35879 "EHLO e36.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755939AbbDIT64 (ORCPT ); Thu, 9 Apr 2015 15:58:56 -0400 Date: Thu, 9 Apr 2015 12:58:50 -0700 From: "Paul E. McKenney" To: Jason Low Cc: Linus Torvalds , Ingo Molnar , 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: <20150409195849.GN6464@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> <1428608618.12911.9.camel@j-VirtualBox> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1428608618.12911.9.camel@j-VirtualBox> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15040919-0021-0000-0000-000009B8F5D6 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2328 Lines: 60 On Thu, Apr 09, 2015 at 12:43:38PM -0700, Jason Low wrote: > 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. Another approach would be to post a timer before entering the spinloop, and have the timer handler set the resched bit. Then the loop would be bounded, safe, and would run at full speed. Thanx, Paul -- 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/