From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753754AbbLKUoW (ORCPT ); Fri, 11 Dec 2015 15:44:22 -0500 Received: from mail-pf0-f182.google.com ([209.85.192.182]:36628 "EHLO mail-pf0-f182.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751486AbbLKUoU (ORCPT ); Fri, 11 Dec 2015 15:44:20 -0500 From: Ani Sinha X-Google-Original-From: Ani Sinha Date: Fri, 11 Dec 2015 12:44:13 -0800 (PST) To: "Paul E. McKenney" cc: Ani Sinha , Rik van Riel , Randy Dunlap , Richard Weinberger , "linux-kernel@vger.kernel.org" , Ivan Delalande , fruggeri Subject: Re: new warning on sysrq kernel crash trigger In-Reply-To: <20151211052647.GL28602@linux.vnet.ibm.com> Message-ID: References: <20151211052647.GL28602@linux.vnet.ibm.com> User-Agent: Alpine 2.00 (OSX 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 Thu, 10 Dec 2015, Paul E. McKenney wrote: > On Thu, Dec 10, 2015 at 03:57:09PM -0800, Ani Sinha wrote: > > Hi guys > > > > I am noticing a new warning in linux 3.18 which we did not see before > > in linux 3.4 : > > > > bash-4.1# echo c > /proc/sysrq-trigger > > [ 978.807185] BUG: sleeping function called from invalid context at > > ../arch/x86/mm/fault.c:1187 > > [ 978.909816] in_atomic(): 0, irqs_disabled(): 0, pid: 4706, name: bash > > [ 978.987358] Preemption disabled at:[] printk+0x48/0x4a > > > > > > I have bisected this to the following change : > > > > commit 984d74a72076a12b400339973e8c98fd2fcd90e5 > > Author: Rik van Riel > > Date: Fri Jun 6 14:38:13 2014 -0700 > > > > sysrq: rcu-ify __handle_sysrq > > > > > > the rcu_read_lock() in handle_sysrq() bumps up > > current->rcu_read_lock_nesting. Hence, in __do_page_fault() when it > > calls might_sleep() in x86/mm/fault.c line 1191, > > preempt_count_equals(0) returns false and hence the warning is > > printed. > > > > One way to handle this would be to do something like this: > > > > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c > > index eef44d9..d4dbe22 100644 > > --- a/arch/x86/mm/fault.c > > +++ b/arch/x86/mm/fault.c > > @@ -1132,7 +1132,7 @@ __do_page_fault(struct pt_regs *regs, unsigned > > long error_code, > > * If we're in an interrupt, have no user context or are running > > * in a region with pagefaults disabled then we must not take the fault > > */ > > - if (unlikely(faulthandler_disabled() || !mm)) { > > + if (unlikely(faulthandler_disabled() || rcu_preempt_depth() || !mm)) { > > This works if CONFIG_PREEMPT=y, but if CONFIG_PREEMPT=n, then > rcu_preempt_depth() unconditionally returns zero. And if > CONFIG_PREEMPT_COUNT=y && CONFIG_PREEMPT=n, you would still see > the might_sleep() splat. > > Maybe use SRCU instead of RCU for this purpose? > >>From ae232ce3fb167b2ad363bfac7aab69001bc55a50 Mon Sep 17 00:00:00 2001 From: Ani Sinha Date: Fri, 11 Dec 2015 12:07:42 -0800 Subject: [PATCH 1/1] Fix 'sleeping function called from invalid context' warning in sysrq generated crash. Commit 984d74a72076a1 ("sysrq: rcu-ify __handle_sysrq") replaced spin_lock_irqsave() calls with rcu_read_lock() calls in sysrq. Since rcu_read_lock() does not disable preemption, faulthandler_disabled() in __do_page_fault() in x86/fault.c returns false. When the code later calls might_sleep() in the pagefault handler, we get the following warning: BUG: sleeping function called from invalid context at ../arch/x86/mm/fault.c:1187 in_atomic(): 0, irqs_disabled(): 0, pid: 4706, name: bash Preemption disabled at:[] printk+0x48/0x4a To fix this, replace RCU call in handle_sysrq() to use SRCU. Tested this patch on linux 3.18 by booting off one of our boards. Fixes: 984d74a72076a1 ("sysrq: rcu-ify __handle_sysrq") Signed-off-by: Ani Sinha --- drivers/tty/sysrq.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c index 5381a72..904865f 100644 --- a/drivers/tty/sysrq.c +++ b/drivers/tty/sysrq.c @@ -519,10 +519,12 @@ void __handle_sysrq(int key, bool check_mask) { struct sysrq_key_op *op_p; int orig_log_level; - int i; + int i, idx; + struct srcu_struct sysrq_rcu; + init_srcu_struct(&sysrq_rcu); rcu_sysrq_start(); - rcu_read_lock(); + idx = srcu_read_lock(&sysrq_rcu); /* * Raise the apparent loglevel to maximum so that the sysrq header * is shown to provide the user with positive feedback. We do not @@ -564,7 +566,7 @@ void __handle_sysrq(int key, bool check_mask) pr_cont("\n"); console_loglevel = orig_log_level; } - rcu_read_unlock(); + srcu_read_unlock(&sysrq_rcu, idx); rcu_sysrq_end(); } -- 1.8.1.4