linux-kernel - Re: [PATCH 1/9] rcu: Panic if RCU tree can not accommodate all CPUs

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Date:	Sat, 7 Mar 2015 13:52:23 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Alexander Gordeev <agordeev@...hat.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/9] rcu: Panic if RCU tree can not accommodate all CPUs

On Sat, Mar 07, 2015 at 06:48:21PM +0000, Alexander Gordeev wrote:
> On Sat, Mar 07, 2015 at 09:42:34AM -0800, Paul E. McKenney wrote:
> > On Sat, Mar 07, 2015 at 06:03:36PM +0100, Alexander Gordeev wrote:
> > > Currently a condition when RCU tree is unable to accommodate
> > > the configured number of CPUs is not permitted and causes
> > > a fall back to compile-time values. However, the code has no
> > > means to exceed the RCU tree capacity neither at compile-time
> > > nor in run-time. Therefore, if the condition is met in run-
> > > time then it indicates a serios problem elsewhere and should
> > > be handled with a panic.
> > > 
> > > Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > > Signed-off-by: Alexander Gordeev <agordeev@...hat.com>
> > 
> > The place to put a check like this is in the code that calculates
> > nr_cpu_ids.  And at least some (perhaps all) are set up so that nr_cpu_ids
> > cannot exceed NR_CPUS, which would render this check redundant.
> 
> The emphasis here the existing check (... n > rcu_capacity[MAX_RCU_LVLS])
> (below as [1]) should not cause the fall back to compiled-time values.
> It either must panic or, as you say - redundant.

You are right, I responded too early on a Saturday.  The point of the
check below is indeed to verify that RCU's calculations are correct.

So do the testing with CONFIG_RCU_FANOUT and CONFIG_RCU_FANOUT_LEAF both
equal to five, and rebase to the rcu/next branch of:

git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git

And I will give them a spin.

							Thanx, Paul

> > So I have to say "no" to this one.
> > 
> > 							Thanx, Paul
> > 
> > > ---
> > >  kernel/rcu/tree.c | 15 +++++++++------
> > >  1 file changed, 9 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index 48d640c..7588c7f 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -3889,16 +3889,19 @@ static void __init rcu_init_geometry(void)
> > >  		rcu_capacity[i] = rcu_capacity[i - 1] * CONFIG_RCU_FANOUT;
> > > 
> > >  	/*
> > > +	 * The tree must be able to accommodate the configured number of CPUs.
> > > +	 * If this limit is exceeded than we have a serious problem elsewhere.
> > > +	 *
> > >  	 * The boot-time rcu_fanout_leaf parameter is only permitted
> > >  	 * to increase the leaf-level fanout, not decrease it.  Of course,
> > >  	 * the leaf-level fanout cannot exceed the number of bits in
> > > -	 * the rcu_node masks.  Finally, the tree must be able to accommodate
> > > -	 * the configured number of CPUs.  Complain and fall back to the
> > > -	 * compile-time values if these limits are exceeded.
> > > +	 * the rcu_node masks.  Complain and fall back to the compile-
> > > +	 * time values if these limits are exceeded.
> > >  	 */
> > > -	if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF ||
> > > -	    rcu_fanout_leaf > sizeof(unsigned long) * 8 ||
> > > -	    n > rcu_capacity[MAX_RCU_LVLS]) {
> 
>             [1]
> 
> > > +	if (n > rcu_capacity[MAX_RCU_LVLS])
> > > +		panic("rcu_init_geometry: rcu_capacity[] is too small");
> > > +	else if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF ||
> > > +		 rcu_fanout_leaf > sizeof(unsigned long) * 8) {
> > >  		WARN_ON(1);
> > >  		return;
> > >  	}
> > > -- 
> > > 1.8.3.1
> > > 
> > 
> 
> -- 
> Regards,
> Alexander Gordeev
> agordeev@...hat.com
> 

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