[PATCH] netdev: fold name hash properly (v3)

[PATCH] netdev: fold name hash properly (v3)

[Stephen Hemminger]

The full_name_hash function does not produce well distributed values in
the lower bits, so most code uses hash_32() to fold it.  This is really
a bug introduced when name hashing was added, back in 2.5 when I added
name hashing.

hash_32 is all that is needed since full_name_hash returns unsigned int
which is only 32 bits on 64 bit platforms.

Also, there is no point in using hash_32 on ifindex, because the is naturally
sequential and usually well distributed.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>

---
Patch against net-next-2.6

--- a/net/core/dev.c	2009-11-09 22:19:08.731481514 -0800
+++ b/net/core/dev.c	2009-11-10 08:42:23.755003727 -0800
@@ -79,6 +79,7 @@
 #include <linux/cpu.h>
 #include <linux/types.h>
 #include <linux/kernel.h>
+#include <linux/hash.h>
 #include <linux/sched.h>
 #include <linux/mutex.h>
 #include <linux/string.h>
@@ -196,7 +197,7 @@ EXPORT_SYMBOL(dev_base_lock);
 static inline struct hlist_head *dev_name_hash(struct net *net, const char *name)
 {
 	unsigned hash = full_name_hash(name, strnlen(name, IFNAMSIZ));
-	return &net->dev_name_head[hash & (NETDEV_HASHENTRIES - 1)];
+	return &net->dev_name_head[hash_32(hash, NETDEV_HASHBITS)];
 }
 
 static inline struct hlist_head *dev_index_hash(struct net *net, int ifindex)

Re: [PATCH] netdev: fold name hash properly (v3)

[Eric Dumazet]

Stephen Hemminger a écrit :
> The full_name_hash function does not produce well distributed values in
> the lower bits, so most code uses hash_32() to fold it.  This is really
> a bug introduced when name hashing was added, back in 2.5 when I added
> name hashing.
> 
> hash_32 is all that is needed since full_name_hash returns unsigned int
> which is only 32 bits on 64 bit platforms.
> 
> Also, there is no point in using hash_32 on ifindex, because the is naturally
> sequential and usually well distributed.
> 
> Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
> 

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>

Thanks Stephen

Re: [PATCH] netdev: fold name hash properly (v3)

[David Miller]

From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Tue, 10 Nov 2009 18:37:11 +0100

> Stephen Hemminger a écrit :
>> The full_name_hash function does not produce well distributed values in
>> the lower bits, so most code uses hash_32() to fold it.  This is really
>> a bug introduced when name hashing was added, back in 2.5 when I added
>> name hashing.
>> 
>> hash_32 is all that is needed since full_name_hash returns unsigned int
>> which is only 32 bits on 64 bit platforms.
>> 
>> Also, there is no point in using hash_32 on ifindex, because the is naturally
>> sequential and usually well distributed.
>> 
>> Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
>> 
> 
> Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>

Applied, thanks.