From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f198.google.com (mail-wr0-f198.google.com [209.85.128.198]) by kanga.kvack.org (Postfix) with ESMTP id 264B76B0006 for ; Sun, 10 Jun 2018 19:57:51 -0400 (EDT) Received: by mail-wr0-f198.google.com with SMTP id r2-v6so11944272wrm.15 for ; Sun, 10 Jun 2018 16:57:51 -0700 (PDT) Received: from mx0a-001b2d01.pphosted.com (mx0b-001b2d01.pphosted.com. [148.163.158.5]) by mx.google.com with ESMTPS id m5-v6si5049388wme.111.2018.06.10.16.57.49 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 10 Jun 2018 16:57:49 -0700 (PDT) Received: from pps.filterd (m0098414.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w5ANsLBl082631 for ; Sun, 10 Jun 2018 19:57:48 -0400 Received: from e12.ny.us.ibm.com (e12.ny.us.ibm.com [129.33.205.202]) by mx0b-001b2d01.pphosted.com with ESMTP id 2jhd5dhc4s-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Sun, 10 Jun 2018 19:57:48 -0400 Received: from localhost by e12.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Sun, 10 Jun 2018 19:57:47 -0400 Date: Sun, 10 Jun 2018 16:59:34 -0700 From: "Paul E. McKenney" Subject: Re: [PATCH v3] mm: fix race between kmem_cache destroy, create and deactivate Reply-To: paulmck@linux.vnet.ibm.com References: <20180530001204.183758-1-shakeelb@google.com> <20180609102027.5vkqucnzvh6nfdxu@esperanza> <20180610163420.GK3593@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Message-Id: <20180610235934.GM3593@linux.vnet.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Shakeel Butt Cc: Vladimir Davydov , Michal Hocko , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Andrew Morton , Greg Thelen , Johannes Weiner , Tejun Heo , Linux MM , Cgroups , LKML On Sun, Jun 10, 2018 at 10:40:17AM -0700, Shakeel Butt wrote: > On Sun, Jun 10, 2018 at 9:32 AM Paul E. McKenney > wrote: > > > > On Sun, Jun 10, 2018 at 07:52:50AM -0700, Shakeel Butt wrote: > > > On Sat, Jun 9, 2018 at 3:20 AM Vladimir Davydov wrote: > > > > > > > > On Tue, May 29, 2018 at 05:12:04PM -0700, Shakeel Butt wrote: > > > > > The memcg kmem cache creation and deactivation (SLUB only) is > > > > > asynchronous. If a root kmem cache is destroyed whose memcg cache is in > > > > > the process of creation or deactivation, the kernel may crash. > > > > > > > > > > Example of one such crash: > > > > > general protection fault: 0000 [#1] SMP PTI > > > > > CPU: 1 PID: 1721 Comm: kworker/14:1 Not tainted 4.17.0-smp > > > > > ... > > > > > Workqueue: memcg_kmem_cache kmemcg_deactivate_workfn > > > > > RIP: 0010:has_cpu_slab > > > > > ... > > > > > Call Trace: > > > > > ? on_each_cpu_cond > > > > > __kmem_cache_shrink > > > > > kmemcg_cache_deact_after_rcu > > > > > kmemcg_deactivate_workfn > > > > > process_one_work > > > > > worker_thread > > > > > kthread > > > > > ret_from_fork+0x35/0x40 > > > > > > > > > > To fix this race, on root kmem cache destruction, mark the cache as > > > > > dying and flush the workqueue used for memcg kmem cache creation and > > > > > deactivation. > > > > > > > > > @@ -845,6 +862,8 @@ void kmem_cache_destroy(struct kmem_cache *s) > > > > > if (unlikely(!s)) > > > > > return; > > > > > > > > > > + flush_memcg_workqueue(s); > > > > > + > > > > > > > > This should definitely help against async memcg_kmem_cache_create(), > > > > but I'm afraid it doesn't eliminate the race with async destruction, > > > > unfortunately, because the latter uses call_rcu_sched(): > > > > > > > > memcg_deactivate_kmem_caches > > > > __kmem_cache_deactivate > > > > slab_deactivate_memcg_cache_rcu_sched > > > > call_rcu_sched > > > > kmem_cache_destroy > > > > shutdown_memcg_caches > > > > shutdown_cache > > > > memcg_deactivate_rcufn > > > > > > > > > > > > Can we somehow flush those pending rcu requests? > > > > > > You are right and thanks for catching that. Now I am wondering if > > > synchronize_sched() just before flush_workqueue() should be enough. > > > Otherwise we might have to replace call_sched_rcu with > > > synchronize_sched() in kmemcg_deactivate_workfn which I would not > > > prefer as that would holdup the kmem_cache workqueue. > > > > > > +Paul > > > > > > Paul, we have a situation something similar to the following pseudo code. > > > > > > CPU0: > > > lock(l) > > > if (!flag) > > > call_rcu_sched(callback); > > > unlock(l) > > > ------ > > > CPU1: > > > lock(l) > > > flag = true > > > unlock(l) > > > synchronize_sched() > > > ------ > > > > > > If CPU0 has called already called call_rchu_sched(callback) then later > > > if CPU1 calls synchronize_sched(). Is there any guarantee that on > > > return from synchronize_sched(), the rcu callback scheduled by CPU0 > > > has already been executed? > > > > No. There is no such guarantee. > > > > You instead want rcu_barrier_sched(), which waits for the callbacks from > > all prior invocations of call_rcu_sched() to be invoked. > > > > Please note that synchronize_sched() is -not- sufficient. It is only > > guaranteed to wait for a grace period, not necessarily for all prior > > callbacks. This goes both directions because if there are no callbacks > > in the system, then rcu_barrier_sched() is within its rights to return > > immediately. > > > > So please make sure you use each of synchronize_sched() and > > rcu_barrier_sched() to do the job that it was intended to do! ;-) > > > > If your lock(l) is shorthand for spin_lock(&l), it looks to me like you > > actually only need rcu_barrier_sched(): > > > > CPU0: > > spin_lock(&l); > > if (!flag) > > call_rcu_sched(callback); > > spin_unlock(&l); > > > > CPU1: > > spin_lock(&l); > > flag = true; > > spin_unlock(&l); > > /* At this point, no more callbacks will be registered. */ > > rcu_barrier_sched(); > > /* At this point, all registered callbacks will have been invoked. */ > > > > On the other hand, if your "lock(l)" was instead shorthand for > > rcu_read_lock_sched(), then you need -both- synchronize_sched() -and- > > rcu_barrier(). And even then, you will be broken in -rt kernels. > > (Which might or might not be a concern, depending on whether your code > > matters to -rt kernels. > > > > Make sense? > > Thanks a lot, that was really helpful. The lock is actually > mutex_lock. So, I think rcu_barrier_sched() should be sufficient. Yes, with either spin_lock() or mutex_lock(), this should work. Mutual exclusion and all that. ;-) Thanx, Paul