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From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, mingo@elte.hu,
	laijs@cn.fujitsu.com, dipankar@in.ibm.com,
	akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca,
	josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de,
	rostedt@goodmis.org, Valdis.Kletnieks@vt.edu,
	dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com,
	fweisbec@gmail.com, sbw@mit.edu, patches@linaro.org
Subject: Re: [PATCH tip/core/rcu 10/15] rcu: Protect rcu_node accesses during CPU stall warnings
Date: Thu, 6 Sep 2012 14:01:12 -0700	[thread overview]
Message-ID: <20120906210112.GB2448@linux.vnet.ibm.com> (raw)
In-Reply-To: <1346943078.18408.29.camel@twins>

On Thu, Sep 06, 2012 at 04:51:18PM +0200, Peter Zijlstra wrote:
> On Thu, 2012-08-30 at 11:56 -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > 
> > The print_other_cpu_stall() function accesses a number of rcu_node
> > fields without protection from the ->lock.  In theory, this is not
> > a problem because the fields accessed are all integers, but in
> > practice the compiler can get nasty.  Therefore, the commit extends
> > the existing critical section to cover the entire loop body.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > ---
> >  kernel/rcutree.c |    6 ++++--
> >  1 files changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index 9f44749..fbe43b0 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -746,14 +746,16 @@ static void print_other_cpu_stall(struct rcu_state *rsp)
> >  	rcu_for_each_leaf_node(rsp, rnp) {
> >  		raw_spin_lock_irqsave(&rnp->lock, flags);
> >  		ndetected += rcu_print_task_stall(rnp);
> > -		raw_spin_unlock_irqrestore(&rnp->lock, flags);
> > -		if (rnp->qsmask == 0)
> > +		if (rnp->qsmask == 0) {
> > +			raw_spin_unlock_irqrestore(&rnp->lock, flags);
> >  			continue;
> > +		}
> >  		for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)
> >  			if (rnp->qsmask & (1UL << cpu)) {
> >  				print_cpu_stall_info(rsp, rnp->grplo + cpu);
> >  				ndetected++;
> >  			}
> > +		raw_spin_unlock_irqrestore(&rnp->lock, flags);
> >  	}
> 
> You now cover printk() and all that that can call with a RCU lock.. is
> this a good thing?

Not particularly good.  However, this happens only if something manages to
block a grace period for 60 seconds, so it should not happen in normal
circumstances.  If that happens, holding the lock allows consistent
state to be printed, which can be helpful in tracking down the bug,
be it in RCU or elsewhere.  So the disease really is worse than the cure.

							Thanx, Paul


  reply	other threads:[~2012-09-06 21:02 UTC|newest]

Thread overview: 86+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-08-30 18:56 [PATCH tip/core/rcu 0/15] Miscellaneous fixes Paul E. McKenney
2012-08-30 18:56 ` [PATCH tip/core/rcu 01/15] rcu: Add PROVE_RCU_DELAY to provoke difficult races Paul E. McKenney
2012-08-30 18:56   ` [PATCH tip/core/rcu 02/15] rcu: Pull TINY_RCU dyntick-idle tracing into non-idle region Paul E. McKenney
2012-08-31 16:53     ` Josh Triplett
2012-08-30 18:56   ` [PATCH tip/core/rcu 03/15] rcu: Properly initialize ->boost_tasks on CPU offline Paul E. McKenney
2012-08-31 17:56     ` Josh Triplett
2012-09-06 14:40     ` Peter Zijlstra
2012-09-06 20:58       ` Paul E. McKenney
2012-08-30 18:56   ` [PATCH tip/core/rcu 04/15] rcu: Permit RCU_NONIDLE() to be used from interrupt context Paul E. McKenney
2012-08-31 18:00     ` Josh Triplett
2012-09-04 22:33       ` Paul E. McKenney
2012-09-04 22:48         ` Josh Triplett
2012-09-04 22:51         ` Steven Rostedt
2012-09-04 23:08           ` Josh Triplett
2012-09-04 23:23             ` Steven Rostedt
2012-09-04 23:33               ` Josh Triplett
2012-09-04 23:43                 ` Paul E. McKenney
2012-09-06 18:54                   ` Josh Triplett
2012-09-06 19:54                     ` Steven Rostedt
2012-09-07  6:09                       ` Josh Triplett
2012-09-07 14:24                         ` Paul E. McKenney
2012-09-07 14:47                           ` Josh Triplett
2012-09-07 15:16                             ` Steven Rostedt
2012-09-12  1:07                               ` Paul E. McKenney
2012-09-12 14:13                                 ` Steven Rostedt
2012-09-12 15:03                                   ` Paul E. McKenney
2012-09-12 15:18                                     ` Steven Rostedt
2012-09-12 16:57                                       ` Paul E. McKenney
2012-09-04 23:46                 ` Steven Rostedt
2012-09-05  0:42                   ` Josh Triplett
2012-09-05  6:23                   ` [PATCH] trace: Don't declare trace_*_rcuidle functions in modules Josh Triplett
2012-09-05 14:26                     ` Mathieu Desnoyers
2012-09-05 16:36                     ` Paul E. McKenney
2012-09-06 19:49                     ` Steven Rostedt
2012-09-14  6:07                     ` [tip:core/rcu] trace: Don' t " tip-bot for Josh Triplett
2012-09-04 23:14           ` [PATCH tip/core/rcu 04/15] rcu: Permit RCU_NONIDLE() to be used from interrupt context Paul E. McKenney
2012-08-30 18:56   ` [PATCH tip/core/rcu 05/15] rcu: Improve boost selection when moving tasks to root rcu_node Paul E. McKenney
2012-08-31 18:09     ` Josh Triplett
2012-08-30 18:56   ` [PATCH tip/core/rcu 06/15] rcu: Make offline-CPU checking allow for indefinite delays Paul E. McKenney
2012-08-31 18:12     ` Josh Triplett
2012-08-30 18:56   ` [PATCH tip/core/rcu 07/15] rcu: Fix obsolete rcu_initiate_boost() header comment Paul E. McKenney
2012-08-31 18:13     ` Josh Triplett
2012-08-30 18:56   ` [PATCH tip/core/rcu 08/15] rcu: Apply for_each_rcu_flavor() to increment_cpu_stall_ticks() Paul E. McKenney
2012-08-31 18:15     ` Josh Triplett
2012-09-04 22:44       ` Paul E. McKenney
2012-08-30 18:56   ` [PATCH tip/core/rcu 09/15] rcu: Avoid rcu_print_detail_task_stall_rnp() segfault Paul E. McKenney
2012-08-31 18:19     ` Josh Triplett
2012-09-04 22:46       ` Paul E. McKenney
2012-09-04 22:55         ` Josh Triplett
2012-08-30 18:56   ` [PATCH tip/core/rcu 10/15] rcu: Protect rcu_node accesses during CPU stall warnings Paul E. McKenney
2012-08-31 18:23     ` Josh Triplett
2012-09-04 22:51       ` Paul E. McKenney
2012-09-06 14:51     ` Peter Zijlstra
2012-09-06 21:01       ` Paul E. McKenney [this message]
2012-08-30 18:56   ` [PATCH tip/core/rcu 11/15] rcu: Avoid spurious RCU " Paul E. McKenney
2012-08-31 18:24     ` Josh Triplett
2012-09-06 14:56     ` Peter Zijlstra
2012-09-06 15:07       ` Steven Rostedt
2012-09-06 15:19         ` Peter Zijlstra
2012-09-06 21:03           ` Paul E. McKenney
2012-09-06 21:41             ` Steven Rostedt
2012-09-06 21:58               ` Paul E. McKenney
2012-09-06 22:05                 ` Steven Rostedt
2012-09-06 22:22                   ` Paul E. McKenney
2012-09-07  7:00                     ` Peter Zijlstra
2012-09-07 14:42                       ` Steven Rostedt
2012-08-30 18:56   ` [PATCH tip/core/rcu 12/15] rcu: Remove redundant memory barrier from __call_rcu() Paul E. McKenney
2012-08-31 18:30     ` Josh Triplett
2012-08-31 18:40       ` Josh Triplett
2012-08-30 18:56   ` [PATCH tip/core/rcu 13/15] rcu: Move TINY_PREEMPT_RCU away from raw_local_irq_save() Paul E. McKenney
2012-08-31 18:34     ` Josh Triplett
2012-09-04 23:03       ` Paul E. McKenney
2012-08-30 18:56   ` [PATCH tip/core/rcu 14/15] time: RCU permitted to stop idle entry via softirq Paul E. McKenney
2012-08-31 18:51     ` Josh Triplett
2012-09-06 15:12     ` Peter Zijlstra
2012-09-06 21:35       ` Paul E. McKenney
2012-09-06 21:57         ` Steven Rostedt
2012-09-06 22:11           ` Paul E. McKenney
2012-08-30 18:56   ` [PATCH tip/core/rcu 15/15] kmemleak: Replace list_for_each_continue_rcu with new interface Paul E. McKenney
2012-08-31 18:55     ` Josh Triplett
2012-09-04 23:41       ` Paul E. McKenney
2012-08-31 16:49   ` [PATCH tip/core/rcu 01/15] rcu: Add PROVE_RCU_DELAY to provoke difficult races Josh Triplett
2012-09-04 22:36     ` Paul E. McKenney
2012-09-06 14:38   ` Peter Zijlstra
2012-09-06 20:51     ` Paul E. McKenney
2012-09-07  6:54       ` Peter Zijlstra

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