From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756652Ab0BKQw7 (ORCPT ); Thu, 11 Feb 2010 11:52:59 -0500 Received: from e4.ny.us.ibm.com ([32.97.182.144]:59345 "EHLO e4.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755755Ab0BKQw6 (ORCPT ); Thu, 11 Feb 2010 11:52:58 -0500 Date: Thu, 11 Feb 2010 08:52:46 -0800 From: "Paul E. McKenney" To: peterz@infradead.org Cc: linux-kernel@vger.kernel.org Subject: rcu_dereference() without protection in select_task_rq_fair() Message-ID: <20100211165246.GA8329@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline 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 Hello, Peter, My lockdep-ified RCU complains about the for_each_domain() in select_task_rq_fair(), see below for the lockdep complaint. I added rcu_dereference_check() annotations as follows: #define for_each_domain_rd(p) \ rcu_dereference_check((p), \ rcu_read_lock_sched_held() || \ lockdep_is_held(&sched_domains_mutex)) #define for_each_domain(cpu, __sd) \ for (__sd = for_each_domain_rd(cpu_rq(cpu)->sd); __sd; __sd = __sd->parent) In other words, I believe (perhaps incorrectly) that for_each_domain() can be called either within an RCU-sched read-side critical section or with sched_domains_mutex held. Lockdep claims that no locks of any kind, RCU or otherwise, were held. I considered the possibility that this was an initialization-time thing, but the code traverses CPU structures rather than task structures. One other possibility is that this is safe due to the fact that we are booting up, before the second CPU has come online. Are you relying on this? For reference, here is the definition of rcu_read_lock_sched_held(): static inline int rcu_read_lock_sched_held(void) { int lockdep_opinion = 0; if (debug_locks) lockdep_opinion = lock_is_held(&rcu_sched_lock_map); return lockdep_opinion || preempt_count() != 0; } Help? Thanx, Paul ============================================== [ BUG: Unsafe rcu_dereference_check() usage! ] ---------------------------------------------- kernel/sched_fair.c:1464 invoked rcu_dereference_check() without protection! other info that might help us debug this: no locks held by swapper/0. stack backtrace: Pid: 0, comm: swapper Not tainted 2.6.33-rc7-autokern1 #1 Call Trace: [] lockdep_rcu_dereference+0x8a/0x92 [] select_task_rq_fair+0xc1/0x686 [] ? __task_pid_nr_ns+0x0/0xb2 [] wake_up_new_task+0x1e/0x13e [] ? __task_pid_nr_ns+0x0/0xb2 [] do_fork+0x2ad/0x329 [] ? kernel_init+0x0/0x199 [] kernel_thread+0x7d/0x8d [] ? kernel_init+0x0/0x199 [] ? kernel_thread_helper+0x0/0x10 [] ? nr_context_switches+0x3f/0x51 [] rest_init+0x21/0xc2 [] start_kernel+0x382/0x38f [] x86_64_start_reservations+0xa9/0xad [] x86_64_start_kernel+0xe6/0xed Setting APIC routing to flat ..TIMER: vector=0x30 apic1=0 pin1=2 apic2=-1 pin2=-1 CPU0: Intel(R) Xeon(R) CPU E5440 @ 2.83GHz stepping 06 lockdep: fixing up alternatives. Booting Node 0, Processors #1lockdep: fixing up alternatives. #2lockdep: fixing up alternatives. #3lockdep: fixing up alternatives. #4lockdep: fixing up alternatives. #5lockdep: fixing up alternatives. #6lockdep: fixing up alternatives. #7 Ok. Brought up 8 CPUs Total of 8 processors activated (45335.31 BogoMIPS). NET: Registered protocol family 16 ACPI: bus type pci registered