Re: sch_generic: warning: the comparison will always evaluate asâtrueâ for the address ofânoop_qdiscâ will never be NULL

From: Eric Dumazet
Date: Sun Aug 14 2011 - 13:40:25 EST

Le dimanche 14 aoÃt 2011 Ã 10:44 -0300, Kevin Winchester a Ãcrit :
> With:
> gcc (GCC) 4.6.1
> I noticed the following warning appearing in my build:
> net/sched/sch_generic.c: In function âdev_graft_qdiscâ:
> net/sched/sch_generic.c:678:2: warning: the comparison will always
> evaluate as âtrueâ for the address of ânoop_qdiscâ will never be NULL
> [-Waddress]
> The code in question runs:
> /* ... and graft new one */
> if (qdisc == NULL)
> qdisc = &noop_qdisc;
> dev_queue->qdisc_sleeping = qdisc;
> rcu_assign_pointer(dev_queue->qdisc, &noop_qdisc);
> where rcu_assign_pointer has a null check that does not apply to
> noop_qdisc, which will never be null.

gcc is a bit stupid here. Of course we know &noop_qdisc cannot be NULL.

> My question is, should that really be assigning &noop_qdisc in there?
> It seems odd to assign &noop_qdisc to qdisc only if qdisc is null, and
> then unconditionally assign &noop_qdisc into dev_queue->qdisc.
> Should the line be:
> rcu_assign_pointer(dev_queue->qdisc, qdisc);
> instead?
> Just curious,

This was already taken into account, the trick is to make rcu_assign()
not trying to be smart, and use RCU_INIT_POINTER() in places we want to
assign NULL pointers.

So one patch is carried by Paul McKenney (RCU maintainer) for next
kernel, and other one in net-next :;a=commitdiff;h=a7590ddfc2c855e75111ef18147a86578fe136e4;a=commitdiff;h=a9b3cd7f323b2e57593e7215362a7b02fc933e3a

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