Date:	Mon, 3 Sep 2012 02:41:26 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu, patches@...aro.org
Subject: Re: [PATCH tip/core/rcu 18/23] rcu: Add random PROVE_RCU_DELAY to
 grace-period initialization

On Thu, Aug 30, 2012 at 11:18:33AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> 
> There are some additional potential grace-period initialization races
> on systems with more than one rcu_node structure, for example:
> 
> 1.	CPU 0 completes a grace period, but needs an additional
> 	grace period, so starts initializing one, initializing all
> 	the non-leaf rcu_node strcutures and the first leaf rcu_node
> 	structure.  Because CPU 0 is both completing the old grace
> 	period and starting a new one, it marks the completion of
> 	the old grace period and the start of the new grace period
> 	in a single traversal of the rcu_node structures.
> 
> 	Therefore, CPUs corresponding to the first rcu_node structure
> 	can become aware that the prior grace period has ended, but
> 	CPUs corresponding to the other rcu_node structures cannot
> 	yet become aware of this.
> 
> 2.	CPU 1 passes through a quiescent state, and therefore informs
> 	the RCU core.  Because its leaf rcu_node structure has already
> 	been initialized, so this CPU's quiescent state is applied to
> 	the new (and only partially initialized) grace period.
> 
> 3.	CPU 1 enters an RCU read-side critical section and acquires
> 	a reference to data item A.  Note that this critical section
> 	will not block the new grace period.
> 
> 4.	CPU 16 exits dyntick-idle mode.  Because it was in dyntick-idle
> 	mode, some other CPU informed the RCU core of its extended
> 	quiescent state for the past several grace periods.  This means
> 	that CPU 16 is not yet aware that these grace periods have ended.
> 
> 5.	CPU 16 on the second leaf rcu_node structure removes data item A
> 	from its enclosing data structure and passes it to call_rcu(),
> 	which queues a callback in the RCU_NEXT_TAIL segment of the
> 	callback queue.
> 
> 6.	CPU 16 enters the RCU core, possibly because it has taken a
> 	scheduling-clock interrupt, or alternatively because it has
> 	more than 10,000 callbacks queued.  It notes that the second
> 	most recent grace period has ended (recall that it cannot yet
> 	become aware that the most recent grace period has completed),
> 	and therefore advances its callbacks.  The callback for data
> 	item A is therefore in the RCU_NEXT_READY_TAIL segment of the
> 	callback queue.
> 
> 7.	CPU 0 completes initialization of the remaining leaf rcu_node
> 	structures for the new grace period, including the structure
> 	corresponding to CPU 16.
> 
> 8.	CPU 16 again enters the RCU core, again, possibly because it has
> 	taken a scheduling-clock interrupt, or alternatively because
> 	it now has more than 10,000 callbacks queued.	It notes that
> 	the most recent grace period has ended, and therefore advances
> 	its callbacks.	The callback for data item A is therefore in
> 	the RCU_NEXT_TAIL segment of the callback queue.
> 
> 9.	All CPUs other than CPU 1 pass through quiescent states, so that
> 	the new grace period completes.  Note that CPU 1 is still in
> 	its RCU read-side critical section, still referencing data item A.
> 
> 10.	Suppose that CPU 2 is the last CPU to pass through a quiescent
> 	state for the new grace period, and suppose further that CPU 2
> 	does not have any callbacks queued.  It therefore traverses
> 	all of the rcu_node structures, marking the new grace period
> 	as completed, but does not initialize a new grace period.
> 
> 11.	CPU 16 yet again enters the RCU core, yet again possibly because
> 	it has taken a scheduling-clock interrupt, or alternatively
> 	because it now has more than 10,000 callbacks queued.	It notes
> 	that the new grace period has ended, and therefore advances
> 	its callbacks.	The callback for data item A is therefore in
> 	the RCU_DONE_TAIL segment of the callback queue.  This means
> 	that this callback is now considered ready to be invoked.
> 
> 12.	CPU 16 invokes the callback, freeing data item A while CPU 1
> 	is still referencing it.
> 
> This sort of scenario represents a day-one bug for TREE_RCU, however,
> the recent changes that permit RCU grace-period initialization to
> be preempted made it much more probable.  Still, it is sufficiently
> improbable to make validation lengthy and inconvenient, so this commit
> adds an anti-heisenbug to greatly increase the collision cross section,
> also known as the probability of occurrence.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

Reviewed-by: Josh Triplett <josh@...htriplett.org>

>  kernel/rcutree.c |    5 +++++
>  1 files changed, 5 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 4cfe488..1373388 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -52,6 +52,7 @@
>  #include <linux/prefetch.h>
>  #include <linux/delay.h>
>  #include <linux/stop_machine.h>
> +#include <linux/random.h>
>  
>  #include "rcutree.h"
>  #include <trace/events/rcu.h>
> @@ -1105,6 +1106,10 @@ static int rcu_gp_init(struct rcu_state *rsp)
>  					    rnp->level, rnp->grplo,
>  					    rnp->grphi, rnp->qsmask);
>  		raw_spin_unlock_irqrestore(&rnp->lock, flags);
> +#ifdef CONFIG_PROVE_RCU_DELAY
> +		if ((random32() % (rcu_num_nodes * 8)) == 0)
> +			schedule_timeout_uninterruptible(2);
> +#endif /* #ifdef CONFIG_PROVE_RCU_DELAY */
>  		cond_resched();
>  	}
>  
> -- 
> 1.7.8
> 
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