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

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Date:	Mon, 9 Mar 2015 21:43:18 -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 04:49:43PM -0700, Paul E. McKenney wrote:
> 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().

As in the following patch.

							Thanx, Paul

------------------------------------------------------------------------

rcu: Shut up spurious gcc uninitialized-variable warning

Because gcc doesn't realize that rcu_num_lvls must be strictly greater
than zero, some versions give a spurious warning about levelcnt[0] being
uninitialized in rcu_init_one().  This commit adds a panic() in that
case in order to educate gcc on this point.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index bd5a9a1db048..5b42d94335e3 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3942,6 +3942,8 @@ static void __init rcu_init_one(struct rcu_state *rsp,
 	/* Silence gcc 4.8 warning about array index out of range. */
 	if (rcu_num_lvls > RCU_NUM_LVLS)
 		panic("rcu_init_one: rcu_num_lvls overflow");
+	if (rcu_num_lvls <= 0)
+		panic("rcu_init_one: rcu_num_lvls underflow");
 
 	/* Initialize the level-tracking arrays. */
 

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