Re: [PATCH v6] hashtable: introduce a small and naive hashtable

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* Sasha Levin (levinsasha928@gmail.com) wrote:
> On 09/26/2012 03:59 PM, Steven Rostedt wrote:
> > On Wed, 2012-09-26 at 14:45 +0100, David Laight wrote:
> >> Amazing how something simple gets lots of comments and versions :-)
> >>
> >>> ...
> >>> + * This has to be a macro since HASH_BITS() will not work on pointers since
> >>> + * it calculates the size during preprocessing.
> >>> + */
> >>> +#define hash_empty(hashtable)							\
> >>> +({										\
> >>> +	int __i;								\
> >>> +	bool __ret = true;							\
> >>> +										\
> >>> +	for (__i = 0; __i < HASH_SIZE(hashtable); __i++)			\
> >>> +		if (!hlist_empty(&hashtable[__i]))				\
> >>> +			__ret = false;						\
> >>> +										\
> >>> +	__ret;									\
> >>> +})
> >>
> >> Actually you could have a #define that calls a function
> >> passing in the address and size.
> > 
> > Probably would be cleaner to do so.
> 
> I think it's worth it if it was more complex than a simple loop. We
> were doing a similar thing with the _size() functions (see version 4
> of this patch), but decided to remove it since it was becoming too
> complex.

Defining local variables within statement-expressions can have some
unexpected side-effects if the "caller" which embeds the macro use the
same variable name. See rcu_dereference() as an example (Paul uses an
awefully large number of underscores). It should be avoided whenever
possible.

> > 
> > 
> >> Also, should the loop have a 'break' in it?
> > 
> > Yeah it should, and could do:
> > 
> > 	for (i = 0; i < HASH_SIZE(hashtable); i++)
> > 		if (!hlist_empty(&hashtable[i]))
> > 			break;
> > 
> > 	return i < HASH_SIZE(hashtable);


Hrm, Steven, did you drink you morning coffee before writing this ? ;-)
It looks like you did 2 bugs in 4 LOC.

First, the condition should be reversed, because this function returns
whether the hash is empty, not the other way around.

And even then, if we would do:

 	for (i = 0; i < HASH_SIZE(hashtable); i++)
 		if (!hlist_empty(&hashtable[i]))
 			break;
 
 	return i >= HASH_SIZE(hashtable);

What happens if the last entry of the table is non-empty ?

So I would advise that Sasha keep his original flag-based
implementation, but add the missing break, and move the init and empty
define loops into static inlines.

Thanks,

Mathieu

> 
> Right.
> 
> 
> Thanks,
> Sasha

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com