From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757335Ab2EBUZv (ORCPT ); Wed, 2 May 2012 16:25:51 -0400 Received: from mail-pb0-f46.google.com ([209.85.160.46]:32845 "EHLO mail-pb0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756655Ab2EBUZu (ORCPT ); Wed, 2 May 2012 16:25:50 -0400 Date: Wed, 2 May 2012 13:25:30 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: "Paul E. McKenney" cc: Benjamin Herrenschmidt , "Paul E. McKenney" , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org Subject: Re: linux-next ppc64: RCU mods cause __might_sleep BUGs In-Reply-To: <20120501232516.GR2441@linux.vnet.ibm.com> Message-ID: References: <1335832418.20866.95.camel@pasglop> <20120501142208.GA2441@linux.vnet.ibm.com> <20120501232516.GR2441@linux.vnet.ibm.com> User-Agent: Alpine 2.00 (LSU 1167 2008-08-23) 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, 1 May 2012, Paul E. McKenney wrote: > > > > > On Mon, 2012-04-30 at 15:37 -0700, Hugh Dickins wrote: > > > > > > > > > > > > BUG: sleeping function called from invalid context at include/linux/pagemap.h:354 > > > > > > in_atomic(): 0, irqs_disabled(): 0, pid: 6886, name: cc1 > > > > > > Call Trace: > > > > > > [c0000001a99f78e0] [c00000000000f34c] .show_stack+0x6c/0x16c (unreliable) > > > > > > [c0000001a99f7990] [c000000000077b40] .__might_sleep+0x11c/0x134 > > > > > > [c0000001a99f7a10] [c0000000000c6228] .filemap_fault+0x1fc/0x494 > > > > > > [c0000001a99f7af0] [c0000000000e7c9c] .__do_fault+0x120/0x684 > > > > > > [c0000001a99f7c00] [c000000000025790] .do_page_fault+0x458/0x664 > > > > > > [c0000001a99f7e30] [c000000000005868] handle_page_fault+0x10/0x30 Got it at last. Embarrassingly obvious. __rcu_read_lock() and __rcu_read_unlock() are not safe to be using __this_cpu operations, the cpu may change in between the rmw's read and write: they should be using this_cpu operations (or, I put preempt_disable/enable in the __rcu_read_unlock below). __this_cpus there work out fine on x86, which was given good instructions to use; but not so well on PowerPC. I've been running successfully for an hour now with the patch below; but I expect you'll want to consider the tradeoffs, and may choose a different solution. Hugh --- 3.4-rc4-next-20120427/include/linux/rcupdate.h 2012-04-28 09:26:38.000000000 -0700 +++ testing/include/linux/rcupdate.h 2012-05-02 11:46:06.000000000 -0700 @@ -159,7 +159,7 @@ DECLARE_PER_CPU(struct task_struct *, rc */ static inline void __rcu_read_lock(void) { - __this_cpu_inc(rcu_read_lock_nesting); + this_cpu_inc(rcu_read_lock_nesting); barrier(); /* Keep code within RCU read-side critical section. */ } --- 3.4-rc4-next-20120427/kernel/rcupdate.c 2012-04-28 09:26:40.000000000 -0700 +++ testing/kernel/rcupdate.c 2012-05-02 11:44:13.000000000 -0700 @@ -72,6 +72,7 @@ DEFINE_PER_CPU(struct task_struct *, rcu */ void __rcu_read_unlock(void) { + preempt_disable(); if (__this_cpu_read(rcu_read_lock_nesting) != 1) __this_cpu_dec(rcu_read_lock_nesting); else { @@ -83,13 +84,14 @@ void __rcu_read_unlock(void) barrier(); /* ->rcu_read_unlock_special load before assign */ __this_cpu_write(rcu_read_lock_nesting, 0); } -#ifdef CONFIG_PROVE_LOCKING +#if 1 /* CONFIG_PROVE_LOCKING */ { int rln = __this_cpu_read(rcu_read_lock_nesting); - WARN_ON_ONCE(rln < 0 && rln > INT_MIN / 2); + BUG_ON(rln < 0 && rln > INT_MIN / 2); } #endif /* #ifdef CONFIG_PROVE_LOCKING */ + preempt_enable(); } EXPORT_SYMBOL_GPL(__rcu_read_unlock); --- 3.4-rc4-next-20120427/kernel/sched/core.c 2012-04-28 09:26:40.000000000 -0700 +++ testing/kernel/sched/core.c 2012-05-01 22:40:46.000000000 -0700 @@ -2024,7 +2024,7 @@ asmlinkage void schedule_tail(struct tas { struct rq *rq = this_rq(); - rcu_switch_from(prev); + /* rcu_switch_from(prev); */ rcu_switch_to(); finish_task_switch(rq, prev); @@ -7093,6 +7093,10 @@ void __might_sleep(const char *file, int "BUG: sleeping function called from invalid context at %s:%d\n", file, line); printk(KERN_ERR + "cpu=%d preempt_count=%x preempt_offset=%x rcu_nesting=%x nesting_save=%x\n", + raw_smp_processor_id(), preempt_count(), preempt_offset, + rcu_preempt_depth(), current->rcu_read_lock_nesting_save); + printk(KERN_ERR "in_atomic(): %d, irqs_disabled(): %d, pid: %d, name: %s\n", in_atomic(), irqs_disabled(), current->pid, current->comm);