Re: [RFC][PATCH] identify in_dev_get rcu read-side critical sections

[Paul E. McKenney]

On Sat, Oct 01, 2005 at 05:12:48PM +1000, Herbert Xu wrote:
> On Fri, Sep 30, 2005 at 11:56:41PM -0700, Suzanne Wood wrote:
> > 
> > But it is interesting to have discarded what was developed yesterday
> > to minimize rcu_dereference impact:
> >
> > >> ----- Original message  -----
> > >> From: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> > >> Date: Fri, 30 Sep 2005 11:19:07 +1000
> > >> 
> > >> On Thu, Sep 29, 2005 at 06:16:03PM -0700, Paul E. McKenney wrote:
> > >>> 
> > >>> OK, how about this instead?
> > >>> 
> > >>> rcu_read_lock();
> > >>> in_dev = dev->ip_ptr;
> > >>> if (in_dev) {
> > >>> atomic_inc(&rcu_dereference(in_dev)->refcnt);
> > >>> }
> > >>> rcu_read_unlock();
> > >>> return in_dev;
> > >> 
> > >> Looks great. 
> > 
> > while adding a function call level by wrapping  __in_dev_get_rcu
> > with in_dev_get as suggested here.
> 
> It might look different, but it should compile to the same result.
> GCC should be smart enough to combine the two branches and produce a
> memory barrier only when in_dev is not NULL.
>  
> > The other thing I'd hoped to address in pktgen.c was 
> > removing the __in_dev_put() which decrements refcnt 
> > while __in_dev_get_rcu() does not increment.
> 
> Well spotted.
> 
> Here is a patch on top of the last one to fix this bogus decrement.

Both this and Herbert's prior patch look good to me!

							Thanx, Paul

> Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> 
> Thanks,
> -- 
> Visit Openswan at http://www.openswan.org/
> Email: Herbert Xu ~{PmV>HI~} <herbert@xxxxxxxxxxxxxxxxxxx>
> Home Page: http://gondor.apana.org.au/~herbert/
> PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

> diff --git a/net/core/pktgen.c b/net/core/pktgen.c
> --- a/net/core/pktgen.c
> +++ b/net/core/pktgen.c
> @@ -1673,7 +1673,6 @@ static void pktgen_setup_inject(struct p
>  					pkt_dev->saddr_min = in_dev->ifa_list->ifa_address;
>  					pkt_dev->saddr_max = pkt_dev->saddr_min;
>  				}
> -				__in_dev_put(in_dev);	
>  			}
>  			rcu_read_unlock();
>  		}

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