Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760040Ab3CZSI2 (ORCPT ); Tue, 26 Mar 2013 14:08:28 -0400 Received: from userp1040.oracle.com ([156.151.31.81]:31555 "EHLO userp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752086Ab3CZSI1 (ORCPT ); Tue, 26 Mar 2013 14:08:27 -0400 Message-ID: <5151E3D2.1070103@oracle.com> Date: Tue, 26 Mar 2013 14:07:14 -0400 From: Sasha Levin User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130310 Thunderbird/17.0.4 MIME-Version: 1.0 To: Davidlohr Bueso CC: Rik van Riel , torvalds@linux-foundation.org, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, hhuang@redhat.com, jason.low2@hp.com, walken@google.com, lwoodman@redhat.com, chegu_vinod@hp.com, "Paul E. McKenney" Subject: Re: ipc,sem: sysv semaphore scalability References: <1363809337-29718-1-git-send-email-riel@surriel.com> <5151DBD3.6080201@oracle.com> <1364320297.5146.7.camel@buesod1.americas.hpqcorp.net> In-Reply-To: <1364320297.5146.7.camel@buesod1.americas.hpqcorp.net> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Source-IP: ucsinet21.oracle.com [156.151.31.93] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2579 Lines: 76 On 03/26/2013 01:51 PM, Davidlohr Bueso wrote: > On Tue, 2013-03-26 at 13:33 -0400, Sasha Levin wrote: >> On 03/20/2013 03:55 PM, Rik van Riel wrote: >>> This series makes the sysv semaphore code more scalable, >>> by reducing the time the semaphore lock is held, and making >>> the locking more scalable for semaphore arrays with multiple >>> semaphores. >> >> Hi Rik, >> >> Another issue that came up is: >> >> [ 96.347341] ================================================ >> [ 96.348085] [ BUG: lock held when returning to user space! ] >> [ 96.348834] 3.9.0-rc4-next-20130326-sasha-00011-gbcb2313 #318 Tainted: G W >> [ 96.360300] ------------------------------------------------ >> [ 96.361084] trinity-child9/7583 is leaving the kernel with locks still held! >> [ 96.362019] 1 lock held by trinity-child9/7583: >> [ 96.362610] #0: (rcu_read_lock){.+.+..}, at: [] SYSC_semtimedop+0x1fb/0xec0 >> >> It seems that we can leave semtimedop without releasing the rcu read lock. >> >> I'm a bit confused by what's going on in semtimedop with regards to rcu read lock, it >> seems that this behaviour is actually intentional? >> >> rcu_read_lock(); >> sma = sem_obtain_object_check(ns, semid); >> if (IS_ERR(sma)) { >> if (un) >> rcu_read_unlock(); >> error = PTR_ERR(sma); >> goto out_free; >> } >> >> When I've looked at that it seems that not releasing the read lock was (very) >> intentional. > > This logic was from the original code, which I also found to be quite > confusing. I wasn't getting this warning with the old code, so there was probably something else that triggers this now. >> >> After that, the only code path that would release the lock starts with: >> >> if (un) { >> ... >> >> So we won't release the lock at all if un is NULL? >> > > Not necessarily, we do release everything at the end of the function: > > out_unlock_free: > sem_unlock(sma, locknum); Ow, there's a rcu_read_unlock() in sem_unlock()? This complicates things even more I suspect. If un is non-NULL we'll be unlocking rcu lock twice? if (un->semid == -1) { rcu_read_unlock(); goto out_unlock_free; } [...] out_unlock_free: sem_unlock(sma, locknum); Thanks, Sasha -- 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/