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

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Message-ID: <20150309233947.GA16995@linux.vnet.ibm.com>
Date:	Mon, 9 Mar 2015 16:39:47 -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 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?

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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