Re: [PATCH ipvs-next] ipvs: Remove rcu_read_unlock();rcu_read_lock();

[Julian Anastasov <ja@xxxxxx>]

        Hello,

On Thu, 25 Apr 2013, Simon Horman wrote:

> It is unclear to me that there is any utility in the following:
> 
>        rcu_read_unlock();
>        rcu_read_lock();

        I thought it is a good idea for fixed hash table
of IP_VS_TAB_BITS=20. May be if guarded by

        if (!((++idx) & 4095))

to reduce its rate to 256 (with idx++ removed from the for loop) ?

        Netfilter has no such logic for nf_conntrack because
it has limit of 16384 rows. Not sure how fatal is to try 1048576
empty rows under RCU lock for such rare operations as
connection listing. OTOH, ip_vs_conn_array() needs to
seek at some initial position, so it can skip many
entries if reading table with many conns, for example,
1048576 rows * 16 conns per row, we will need to
touch 16777216 conns under lock. Not sure what is the
best practice for such cases.

> So this patch removes the two instances of the above from ip_vs_conn.c.
> 
> This was introduced in 7cf2eb7bccbe0d7a8ab1d1382c4faa2b3abf817f ("ipvs: fix
> sparse warnings for ip_vs_conn listing") as part of cleaning up warnings
> flagged by sparse.
> 
> Compile and sparse tested only.
> 
> Cc: Julian Anastasov <ja@xxxxxx>
> Reported-by: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
> Signed-off-by: Simon Horman <horms@xxxxxxxxxxxx>
> ---
>  net/netfilter/ipvs/ip_vs_conn.c | 4 ----
>  1 file changed, 4 deletions(-)
> 
> diff --git a/net/netfilter/ipvs/ip_vs_conn.c b/net/netfilter/ipvs/ip_vs_conn.c
> index a083bda..458dc68 100644
> --- a/net/netfilter/ipvs/ip_vs_conn.c
> +++ b/net/netfilter/ipvs/ip_vs_conn.c
> @@ -975,8 +975,6 @@ static void *ip_vs_conn_array(struct seq_file *seq, 
> loff_t pos)
>                               return cp;
>                       }
>               }
> -             rcu_read_unlock();
> -             rcu_read_lock();
>       }
>  
>       return NULL;
> @@ -1015,8 +1013,6 @@ static void *ip_vs_conn_seq_next(struct seq_file *seq, 
> void *v, loff_t *pos)
>                       iter->l = &ip_vs_conn_tab[idx];
>                       return cp;
>               }
> -             rcu_read_unlock();
> -             rcu_read_lock();
>       }
>       iter->l = NULL;
>       return NULL;
> -- 
> 1.8.2.1

Regards

--
Julian Anastasov <ja@xxxxxx>
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