From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758504AbYE0PDr (ORCPT ); Tue, 27 May 2008 11:03:47 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757398AbYE0PD3 (ORCPT ); Tue, 27 May 2008 11:03:29 -0400 Received: from smtp1.linux-foundation.org ([140.211.169.13]:58242 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757204AbYE0PD2 (ORCPT ); Tue, 27 May 2008 11:03:28 -0400 Date: Tue, 27 May 2008 08:03:03 -0700 (PDT) From: Linus Torvalds To: Oleg Nesterov cc: Alexey Dobriyan , Linux Kernel Mailing List , Andrew Morton , "Paul E. McKenney" Subject: Re: 2.6.26-rc4: RIP find_pid_ns+0x6b/0xa0 In-Reply-To: <20080527090605.GA146@tv-sign.ru> Message-ID: References: <20080527052318.GA28301@martell.zuzino.mipt.ru> <20080527090605.GA146@tv-sign.ru> User-Agent: Alpine 1.10 (LFD 962 2008-03-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 27 May 2008, Oleg Nesterov wrote: > On 05/27, Alexey Dobriyan wrote: > > > > PREEMPT_RCU is in use, again. I do wonder if PREEMPT_RCU is broken. > > 0xffffffff802447cb is in find_pid_ns (kernel/pid.c:297). > > 292 struct hlist_node *elem; > > 293 struct upid *pnr; > > 294 > > 295 hlist_for_each_entry_rcu(pnr, elem, > > 296 &pid_hash[pid_hashfn(nr, ns)], pid_chain) > > 297 if (pnr->nr == nr && pnr->ns == ns) > > general protection fault: 0000 [2] PREEMPT SMP DEBUG_PAGEALLOC > > RDX: 6b6b6b6b6b6b6b6b RSI: ffffffff80566760 RDI: 0000000000003cef That repeated 0x6b is POISON_FREE, and the code is cmp -0x10(%rdx),%edi which is the load of "pnr->nr". So 'pnr' has been free'd. On Tue, 27 May 2008, Oleg Nesterov wrote: > > Is this reproducible? > > In theory find_pid() is not safe without rcu_read_lock() if CONFIG_PREEMPT_RCU. > But we have a lot of "read_lock(tasklist_lock) + find_pid()", this was legal > and documented. It was actually broken, but happened to work because read_lock() > implied rcu_read_lock(). > > Could you look at > > [PATCH] fix tasklist + find_pid() with CONFIG_PREEMPT_RCU > http://marc.info/?t=120162615300012 > > ? > > I am not sure this is the actual reason though, the race is very unlikely. That is a *very* unlikely race, especially as that bad_fork_free_pid case would only happen if pid_ns_prepare_proc() fails. And if it fails, it's still very unlikely to hit, I think. That said, it does smell like a bug. But I *really* would be much much happier if even SRCU at least waited for a grace period, so that it would always be safe to just disable preemption for a "rcu_read_lock()". That way, things that take spinlocks are safe even with SRCU. Paul? How hard would it be to make preemptable RCU just honor that classic RCU behavior? Linus