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

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Submitter Simon Horman
Date April 25, 2013, 1:12 a.m.
Message ID <1366852353-32561-1-git-send-email-horms@verge.net.au>
Download mbox | patch
Permalink /patch/239368/
State Not Applicable
Headers show

Comments

Simon Horman - April 25, 2013, 1:12 a.m.
It is unclear to me that there is any utility in the following:

	 rcu_read_unlock();
	 rcu_read_lock();

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@ssi.bg>
Reported-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Simon Horman <horms@verge.net.au>
---
 net/netfilter/ipvs/ip_vs_conn.c | 4 ----
 1 file changed, 4 deletions(-)
Julian Anastasov - April 25, 2013, 8:15 a.m.
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@ssi.bg>
> Reported-by: Pablo Neira Ayuso <pablo@netfilter.org>
> Signed-off-by: Simon Horman <horms@verge.net.au>
> ---
>  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@ssi.bg>
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Hans Schillstrom - April 25, 2013, 9:05 a.m.
Hello
On Thu, 2013-04-25 at 11:15 +0300, Julian Anastasov wrote:
> 	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.

My opinion is to keep it, people tends to do such "rare" things.
It's not unusual with 256k - 1M rows... 

Regards 
Hans
Simon Horman - April 25, 2013, 1:36 p.m.
On Thu, Apr 25, 2013 at 11:05:26AM +0200, Hans Schillstrom wrote:
> Hello
> On Thu, 2013-04-25 at 11:15 +0300, Julian Anastasov wrote:
> > 	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.
> 
> My opinion is to keep it, people tends to do such "rare" things.
> It's not unusual with 256k - 1M rows... 

Ok, leaving it seems reasonable.
Pablo, do you have any objections?

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Eric Dumazet - April 25, 2013, 2:04 p.m.
On Thu, 2013-04-25 at 22:36 +0900, Simon Horman wrote:

> Ok, leaving it seems reasonable.
> Pablo, do you have any objections?

I have objections.

I would _add_ a cond_resched() there to explicitly do what we want

Maybe a macro/inline doing this already exists.

static void inline cond_resched_rcu_lock(void)
{
	if (need_resched()) {
		rcu_read_unlock();
		cond_resched();
		rcu_read_lock();
	}
}





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Julian Anastasov - April 25, 2013, 7:46 p.m.
Hello,

On Thu, 25 Apr 2013, Eric Dumazet wrote:

> On Thu, 2013-04-25 at 22:36 +0900, Simon Horman wrote:
> 
> > Ok, leaving it seems reasonable.
> > Pablo, do you have any objections?
> 
> I have objections.
> 
> I would _add_ a cond_resched() there to explicitly do what we want
> 
> Maybe a macro/inline doing this already exists.
> 
> static void inline cond_resched_rcu_lock(void)
> {
> 	if (need_resched()) {
> 		rcu_read_unlock();
> 		cond_resched();
> 		rcu_read_lock();
> 	}
> }

	Thanks, looks like a good idea to me, I guess
its place is include/linux/sched.h. Simon, can you prepare
2 patches instead, one for cond_resched_rcu_lock and second
for ipvs?

Regards

--
Julian Anastasov <ja@ssi.bg>
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Simon Horman - April 26, 2013, 12:28 a.m.
On Thu, Apr 25, 2013 at 10:46:55PM +0300, Julian Anastasov wrote:
> 
> 	Hello,
> 
> On Thu, 25 Apr 2013, Eric Dumazet wrote:
> 
> > On Thu, 2013-04-25 at 22:36 +0900, Simon Horman wrote:
> > 
> > > Ok, leaving it seems reasonable.
> > > Pablo, do you have any objections?
> > 
> > I have objections.
> > 
> > I would _add_ a cond_resched() there to explicitly do what we want
> > 
> > Maybe a macro/inline doing this already exists.
> > 
> > static void inline cond_resched_rcu_lock(void)
> > {
> > 	if (need_resched()) {
> > 		rcu_read_unlock();
> > 		cond_resched();
> > 		rcu_read_lock();
> > 	}
> > }
> 
> 	Thanks, looks like a good idea to me, I guess
> its place is include/linux/sched.h. Simon, can you prepare
> 2 patches instead, one for cond_resched_rcu_lock and second
> for ipvs?

Sure, will do.
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Pablo Neira - April 26, 2013, 12:59 a.m.
Hi Julian,

On Thu, Apr 25, 2013 at 11:15:25AM +0300, Julian Anastasov wrote:
> 
> 	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.

We seem to be supporting over that limit via module_param and sysfs:

/sys/module/nf_conntrack/parameters/hashsize

Regards.
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Julian Anastasov - April 26, 2013, 6:02 a.m.
Hello,

On Fri, 26 Apr 2013, Pablo Neira Ayuso wrote:

> Hi Julian,
> 
> On Thu, Apr 25, 2013 at 11:15:25AM +0300, Julian Anastasov wrote:
> > 
> > 	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.
> 
> We seem to be supporting over that limit via module_param and sysfs:
> 
> /sys/module/nf_conntrack/parameters/hashsize

	Thanks for the note, I overlooked this parameter.

Regards

--
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Patch

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;