From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754017Ab2DQQpo (ORCPT ); Tue, 17 Apr 2012 12:45:44 -0400 Received: from e39.co.us.ibm.com ([32.97.110.160]:54623 "EHLO e39.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751241Ab2DQQpm (ORCPT ); Tue, 17 Apr 2012 12:45:42 -0400 Date: Tue, 17 Apr 2012 09:45:03 -0700 From: "Paul E. McKenney" To: Sasha Levin Cc: Dave Jones , "linux-kernel@vger.kernel.org List" Subject: Re: New RCU related warning due to rcu_preempt_depth() changes Message-ID: <20120417164503.GA27031@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20120417150556.GB2404@linux.vnet.ibm.com> <20120417155316.GE2404@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20120417155316.GE2404@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12041716-4242-0000-0000-0000015F85FB Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 17, 2012 at 08:53:16AM -0700, Paul E. McKenney wrote: > On Tue, Apr 17, 2012 at 05:36:59PM +0200, Sasha Levin wrote: > > On Tue, Apr 17, 2012 at 5:05 PM, Paul E. McKenney > > wrote: > > > On Tue, Apr 17, 2012 at 10:42:47AM +0200, Sasha Levin wrote: > > >> Hi Paul, > > >> > > >> It looks like commit 7298b03 ("rcu: Move __rcu_read_lock() and > > >> __rcu_read_unlock() to per-CPU variables") is causing the following > > >> warning (I've added the extra fields on the second line): > > >> > > >> [   77.330920] BUG: sleeping function called from invalid context at > > >> mm/memory.c:3933 > > >> [   77.336571] in_atomic(): 0, irqs_disabled(): 0, preempt count: 0, > > >> preempt offset: 0, rcu depth: 1, pid: 5669, name: trinity > > >> [   77.344135] no locks held by trinity/5669. > > >> [   77.349644] Pid: 5669, comm: trinity Tainted: G        W > > >> 3.4.0-rc3-next-20120417-sasha-dirty #83 > > >> [   77.354401] Call Trace: > > >> [   77.355956]  [] __might_sleep+0x1f3/0x210 > > >> [   77.358811]  [] might_fault+0x2f/0xa0 > > >> [   77.361997]  [] schedule_tail+0x88/0xb0 > > >> [   77.364671]  [] ret_from_fork+0x13/0x80 > > >> > > >> As you can see, rcu_preempt_depth() returns 1 when running in that > > >> context, which looks pretty odd. > > > > > > Ouch!!! > > > > > > So it looks like I missed a place where I need to save and restore > > > the new per-CPU rcu_read_lock_nesting and rcu_read_unlock_special > > > variables.  My (probably hopelessly naive) guess is that I need to add > > > a rcu_switch_from() and rcu_switch_to() into schedule_tail(), but to > > > make rcu_switch_from() take the task_struct pointer as an argument, > > > passing in prev. > > > > > > Does this make sense, or am I still missing something here? > > > > I've let the test run for a bit more, and it appears that I'm getting > > this warning from lots of different sources, would this > > schedule_tail() fix all of them? > > If I understand the failure correctly, yes. If the task switches without > RCU paying attention, the nesting count for both the outgoing and the > incoming tasks can get messed up. The messed-up counts could easily > cause problems downstream. > > Of course, there might well be additional bugs. > > I will put a speculative patch together and send it along. And here it is, testing just started. Thanx, Paul ------------------------------------------------------------------------ rcu: Add RCU context switching to schedule_tail() The new rcu_read_lock_nesting and rcu_read_unlock_special per-CPU variables must be saved and restored at every context switch, including those involving schedule_tail(). This commit therefore adds the saving and restoring to schedul_tail(). Reported-by: Sasha Levin Signed-off-by: Paul E. McKenney Signed-off-by: Paul E. McKenney diff --git a/arch/um/drivers/mconsole_kern.c b/arch/um/drivers/mconsole_kern.c index 25a7fea..32272d4 100644 --- a/arch/um/drivers/mconsole_kern.c +++ b/arch/um/drivers/mconsole_kern.c @@ -704,7 +704,7 @@ static void stack_proc(void *arg) struct task_struct *from = current, *to = arg; to->thread.saved_task = from; - rcu_switch_from(); + rcu_switch_from(from); switch_to(from, to, from); rcu_switch_to(); } diff --git a/include/linux/sched.h b/include/linux/sched.h index f2468cb..0d48609 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1937,10 +1937,8 @@ static inline void rcu_copy_process(struct task_struct *p) * * The caller must have disabled preemption. */ -static inline void rcu_switch_from(void) +static inline void rcu_switch_from(struct task_struct *t) { - struct task_struct *t = current; - if (__this_cpu_read(rcu_read_lock_nesting) != 0) rcu_preempt_note_context_switch(); t->rcu_read_lock_nesting_save = __this_cpu_read(rcu_read_lock_nesting); @@ -1991,7 +1989,7 @@ static inline void rcu_copy_process(struct task_struct *p) { } -static inline void rcu_switch_from(void) +static inline void rcu_switch_from(struct task_struct *t) { } diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 51ce537..17ae267 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2024,6 +2024,8 @@ asmlinkage void schedule_tail(struct task_struct *prev) { struct rq *rq = this_rq(); + rcu_switch_from(prev); + rcu_switch_to(); finish_task_switch(rq, prev); /* @@ -2083,7 +2085,7 @@ context_switch(struct rq *rq, struct task_struct *prev, #endif /* Here we just switch the register state and the stack. */ - rcu_switch_from(); + rcu_switch_from(current); switch_to(prev, next, prev); rcu_switch_to();