From: "Paul E. McKenney" <paulmck@linux•vnet.ibm.com>
To: Eric Dumazet <eric.dumazet@gmail•com>
Cc: Cong Wang <cwang@twopensource•com>,
Tom Herbert <therbert@google•com>,
netdev <netdev@vger•kernel.org>
Subject: Re: suspicious RCU usage in net/ipv4/ip_tunnel.c:80
Date: Mon, 13 Jan 2014 20:53:16 -0800 [thread overview]
Message-ID: <20140114045316.GV10038@linux.vnet.ibm.com> (raw)
In-Reply-To: <1389601233.31367.213.camel@edumazet-glaptop2.roam.corp.google.com>
On Mon, Jan 13, 2014 at 12:20:33AM -0800, Eric Dumazet wrote:
> On Sun, 2014-01-12 at 22:36 -0800, Cong Wang wrote:
> > > Please read rcu_dereference_protected() documentation in
> > > include/linux/rcupdate.h
> >
> > I did before I replied.
>
>
>
> >
> > >
> > > Also you can run sparse, with CONFIG_SPARSE_RCU_POINTER=y in
> > > your .config
> > >
> > > make C=2 net/ipv4/ip_tunnel.o
> > >
> > > And then you'll know the answer to this question.
> > >
> >
> > Sounds like it is only to shut up a sparse warning, then its name
> > is misleading, we clearly don't dereference it here.
OK, I'll bite... This code invokes dst_release() which looks to me
like it dereferences this pointer:
void dst_release(struct dst_entry *dst)
{
if (dst) {
int newrefcnt;
newrefcnt = atomic_dec_return(&dst->__refcnt);
WARN_ON(newrefcnt < 0);
if (unlikely(dst->flags & DST_NOCACHE) && !newrefcnt) {
dst = dst_destroy(dst);
if (dst)
__dst_free(dst);
}
}
}
If you really were not dereferencing the pointer, for example, if you
were only testing it against NULL or some such, then you can use
rcu_access_pointer(). This is a bit cheaper on DEC Alpha, FWIW.
You can think of rcu_dereference() as meaning "fetch this RCU-protected
pointer with intent to dereference()" if that helps.
Or am I missing your point?
> Historical reasons, you should have been there when Paul invented the
> name and lazy people like us let him do so !
>
> http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=b62730baea32f86fe91a7930e4b7ee8d82778b79
Indeed! ;-)
And rcu_dereference() is at least an improvement over the earlier
hand-placed smp_read_barrier_depends().
Thanx, Paul
> You are lucky, there is plenty of documentation, maybe too much..
>
>
>
> --
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>
next prev parent reply other threads:[~2014-01-14 4:53 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-01-11 0:37 suspicious RCU usage in net/ipv4/ip_tunnel.c:80 Cong Wang
2014-01-11 1:21 ` Eric Dumazet
2014-01-11 19:15 ` Cong Wang
2014-01-12 17:44 ` Eric Dumazet
2014-01-13 6:36 ` Cong Wang
2014-01-13 8:20 ` Eric Dumazet
2014-01-14 4:53 ` Paul E. McKenney [this message]
2014-01-17 0:26 ` Cong Wang
2014-01-17 0:49 ` Eric Dumazet
2014-01-13 19:35 ` Cong Wang
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