linux-kernel - Re: [PATCH v2 1/9] rcu: Panic if RCU tree can not accommodate all CPUs

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Message-ID: <20150602064527.GA3679@agordeev.usersys.redhat.com>
Date:	Tue, 2 Jun 2015 07:45:27 +0100
From:	Alexander Gordeev <agordeev@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2 1/9] rcu: Panic if RCU tree can not accommodate all
 CPUs

On Mon, Jun 01, 2015 at 11:37:21AM -0700, Paul E. McKenney wrote:
> On Fri, May 29, 2015 at 11:53:37AM +0200, 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>
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > Signed-off-by: Alexander Gordeev <agordeev@...hat.com>
> > ---
> >  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 2fce662..66a4230 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -4117,16 +4117,19 @@ static void __init rcu_init_geometry(void)
> >  		rcu_capacity[i] = rcu_capacity[i - 1] * 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 < RCU_FANOUT_LEAF ||
> > -	    rcu_fanout_leaf > sizeof(unsigned long) * 8 ||
> > -	    n > rcu_capacity[MAX_RCU_LVLS]) {
> > +	if (n > rcu_capacity[MAX_RCU_LVLS])
> > +		panic("rcu_init_geometry: rcu_capacity[] is too small");
> 
> The way this is set up, if the boot parameter (illegally) sets
> rcu_fanout_lead smaller than RCU_FANOUT_LEAF, we might panic.  It would
> be far better to first check for rcu_fanout_leaf being out of bounds,
> and only then have the possibility of panic().  That way, a typo in
> the rcu_fanout_leaf boot paremeter is ignored, but with a splat.
> 
> Or am I missing something here?

I think you are quite right. But the bounds check is misplaced then.
I would say, it should be placed before rcu_capacity[] seed, as it
only deals with constants and has nothing with rcu_capacity[].

I will send the updated version.

> 							Thanx, Paul
> 
> > +	else if (rcu_fanout_leaf < RCU_FANOUT_LEAF ||
> > +		 rcu_fanout_leaf > sizeof(unsigned long) * 8) {
> >  		WARN_ON(1);
> >  		return;
> >  	}
> > -- 
> > 1.8.3.1
> > 
> 
> --
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-- 
Regards,
Alexander Gordeev
agordeev@...hat.com
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