From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758592AbZBTPqc (ORCPT ); Fri, 20 Feb 2009 10:46:32 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753531AbZBTPqX (ORCPT ); Fri, 20 Feb 2009 10:46:23 -0500 Received: from e2.ny.us.ibm.com ([32.97.182.142]:36361 "EHLO e2.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753397AbZBTPqW (ORCPT ); Fri, 20 Feb 2009 10:46:22 -0500 Date: Fri, 20 Feb 2009 07:46:19 -0800 From: "Paul E. McKenney" To: Vegard Nossum Cc: Ingo Molnar , stable@kernel.org, Andrew Morton , Nick Piggin , Pekka Enberg , linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: fix lazy vmap purging (use-after-free error) Message-ID: <20090220154619.GC6960@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20090220134121.GA19575@damson.getinternet.no> <20090220135000.GA9616@elte.hu> <20090220140157.GA12799@elte.hu> <19f34abd0902200651k7e86aebay5398ef5ac0578561@mail.gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <19f34abd0902200651k7e86aebay5398ef5ac0578561@mail.gmail.com> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 20, 2009 at 03:51:28PM +0100, Vegard Nossum wrote: > 2009/2/20 Ingo Molnar : > > > > * Ingo Molnar wrote: > > > >> ah, indeed: > >> > >> list_del_rcu(&va->list); > >> > >> i suspect it could be hit big time in a workload that opens > >> more than 512 files, as expand_files() uses a > >> vmalloc()+vfree() pair in that case. > > > > hm, perhaps it's not a problem after all. The freeing is done > > via rcu, and list_del_rcu() leaves the forward pointer intact. > > Well, it's not the particular line that you posted, in any case. > That's &va->list, but the traversed list is &va->purge_list. > > I thought it would be the line: > > call_rcu(&va->rcu_head, rcu_free_va); > > (which does kfree() in the callback) that was the problem. > > > So how did it happen that the entry got kfree()d before the loop > > was done? We are in a spinlocked section so the CPU should not > > have entered rcu processing. > > I added some printks to __free_vmap_area() and rcu_free_va(), and it > shows that the kfree() is being called immediately (inside the list > traversal). So the call_rcu() is happening immediately (or almost > immediately). > > If I've understood correctly, the RCU processing can happen inside a > spinlock, as long as interrupts are enabled. (Won't the timer IRQ > trigger softirq processing, which triggers RCU callback processing, > for example?) > > And interrupts are enabled when this happens: EFLAGS: 00000292 > > Please correct me if I am wrong! If you are using preemptable RCU, and if the read side accesses are not protected by rcu_read_lock(), this can happen. At least for values of "immediately" in the millisecond range. If you were using classic or hierarchical RCU, the fact that the call_rcu() is within a spinlock (as opposed to mutex) critical section should prevent the grace period from ending. So, what flavor of RCU were you using? Thanx, Paul