linux-kernel - Re: [PATCH] rcu: shift by 1UL rather than 1 to fix sign extension error

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Date:   Tue, 13 Dec 2016 07:05:35 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Colin King <colin.king@...onical.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: shift by 1UL rather than 1 to fix sign extension
 error

On Tue, Dec 13, 2016 at 08:25:42PM +0800, Boqun Feng wrote:
> On Tue, Dec 13, 2016 at 07:21:48PM +0800, Boqun Feng wrote:
> > On Tue, Dec 13, 2016 at 10:56:46AM +0000, Colin King wrote:
> > > From: Colin Ian King <colin.king@...onical.com>
> > > 
> > > mask and bit are unsigned longs, so if bit is 31 we end up sign
> > > extending the 1 and mask ends up as 0xffffffff80000000. Fix this
> > > by explicitly adding integer suffix UL ensure 1 is a unsigned long
> > > rather than an signed int.
> > > 
> > 
> > Right, you are, and the tool is ;-)
> > 
> > If @bit is greater than 32, we even got an undefined behavior in C ;-(
> > This is my careless mistake, thank you for finding it out and fix it!
> > 
> > > Issue found with static analysis with CoverityScan, CID 1388564
> > > 
> > > Fixes: 8965c3ce4718754db ("rcu: Use leaf_node_for_each_mask_possible_cpu() in force_qs_rnp()")
> > > Signed-off-by: Colin Ian King <colin.king@...onical.com>
> > 
> > I think Paul only queued that for running tests and I have almost
> > finished a v2. I will fold your fix in my patch and add your SoB along
> > with mine, does that work for you?
> > 
> > TBH, this situation is kinda new to me, so if anyone has any suggestion,
> > please let me know ;-)
> > 
> > Regards,
> > Boqun
> > 
> > > ---
> > >  kernel/rcu/tree.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index 10162ac..6ecedd8 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -3051,7 +3051,7 @@ static void force_qs_rnp(struct rcu_state *rsp,
> > >  
> > >  		leaf_node_for_each_mask_possible_cpu(rnp, rnp->qsmask, bit, cpu)
> > >  			if (f(per_cpu_ptr(rsp->rda, cpu), isidle, maxj))
> > > -				mask |= 1 << bit;
> > > +				mask |= 1UL << bit;
> 
> Hmm.. Seems using BIT() here is a good idea, and maybe rename bit as
> grp_idx or something.

Well, "bit" could be a bit, or it could be the number of the bit.
Given that we have "mask", and given that we are shifting by it,
it has to be the number of the bit.

> Naming, naming, naming..

Often I think that the biggest problem with naming is putting too much
time into worrying about it.  ;-)

But if you want to change the name from "bit" to "bitno" or "bitnum"
as part of your updated patchset, I won't object.  The reason for
preferring either to "grp_idx" is that "bitno" goes well with "mask".
But as a general rule, I must follow the usual practice of not favoring
renaming patches.

							Thanx, Paul

> Regards,
> Boqun
> 
> > >  
> > >  		if (mask != 0) {
> > >  			/* Idle/offline CPUs, report (releases rnp->lock. */
> > > -- 
> > > 2.10.2
> > > 
> 
>