linux-kernel - Re: [PATCH 00/10] rcu: Cleanup RCU tree initialization

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Date:	Mon, 9 Mar 2015 16:49:43 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Alexander Gordeev <agordeev@...hat.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/10] rcu: Cleanup RCU tree initialization

On Mon, Mar 09, 2015 at 04:39:47PM -0700, Paul E. McKenney wrote:
> On Mon, Mar 09, 2015 at 02:40:21PM -0700, Paul E. McKenney wrote:
> > On Mon, Mar 09, 2015 at 09:34:04AM +0100, Alexander Gordeev wrote:
> > > Hi Paul,
> > > 
> > > Here is cleanup of RCU tree initialization rebased on linux-rcu rcu/next
> > > repo, as you requested. Please, note an extra patch #10 that was not
> > > present in the first post.
> > > 
> > > The series successfully passes kernel build test with CONFIG_RCU_FANOUT
> > > and CONFIG_RCU_FANOUT_LEAF equal to 5.
> > 
> > I queued up 1-9, as discussed and have started testing.  I will let you
> > know how it goes.
> 
> Initial testing went well except for the following warning:
> 
> /home/paulmck/public_git/linux-rcu/kernel/rcu/tree.c: In function ‘rcu_init_one.isra.63’:
> /home/paulmck/public_git/linux-rcu/kernel/rcu/tree.c:3961:3: warning: ‘levelcnt[0]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
>    for (j = 0; j < levelcnt[i]; j++, rnp++) {
> 
> This warning looks like a false positive to me, given that the loop
> near the beginning of the function initializes levelcnt[0].  Am I
> missing something here, and either way, what is the best way to shut
> this warning up?

My suggestion is the following:

	if (rcu_num_lvls <= 0)
		panic("rcu_init_one: rcu_num_lvls underflow");

Just following the other panic() in rcu_init_one().

							Thanx, Paul

> I am using gcc version 4.8.2.
> 
> This warning only appears for some configurations.
> SRCU-N:  Four CPUs, single rcu_node structure.
> SRCU-P:  Eight CPUs, single rcu_node structure.
> TASKS01: Two CPUs, single rcu_node structure.
> TASKS03: Two CPUs, single rcu_node structure.
> TREE09:  Single CPU, single rcu_node structure.
> 
> All the other TREE0N configurations have multiple rcu_node structures.
> 
> TASKS02 didn't warn either despite having only one CPU, but that is
> because it uses Tiny RCU instead of Tree RCU.
> 
> So this warning shows up only in the case where the compiler knows
> that the rcu_node tree consists of only one rcu_node structure.
> 
> 							Thanx, Paul
> 
> > > Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > > 
> > > 
> > > Alexander Gordeev (10):
> > >   rcu: Panic if RCU tree can not accommodate all CPUs
> > >   rcu: Remove superfluous local variable in rcu_init_geometry()
> > >   rcu: Cleanup rcu_init_geometry() code and arithmetics
> > >   rcu: Simplify rcu_init_geometry() capacity arithmetics
> > >   rcu: Limit rcu_state::levelcnt[] to RCU_NUM_LVLS items
> > >   rcu: Limit rcu_capacity[] size to RCU_NUM_LVLS items
> > >   rcu: Remove unnecessary fields from rcu_state structure
> > >   rcu: Limit count of static data to the number of RCU levels
> > >   rcu: Simplify arithmetic to calculate number of RCU nodes
> > >   rcu: Remove unnecessary grpnum field from rcu_node structure
> > > 
> > >  kernel/rcu/tree.c        | 102 ++++++++++++++++++++---------------------------
> > >  kernel/rcu/tree.h        |  35 ++++++++--------
> > >  kernel/rcu/tree_plugin.h |   4 +-
> > >  3 files changed, 63 insertions(+), 78 deletions(-)
> > > 
> > > -- 
> > > 1.8.3.1
> > > 

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