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

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Date:	Wed, 26 Sep 2012 12:09:24 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Sasha Levin <levinsasha928@...il.com>,
	David Laight <David.Laight@...LAB.COM>,
	torvalds@...ux-foundation.org, tj@...nel.org,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	ebiederm@...ssion.com, neilb@...e.de, bfields@...ldses.org,
	ejt@...hat.com, snitzer@...hat.com, edumazet@...gle.com,
	josh@...htriplett.org, rmallon@...il.com, palves@...hat.com
Subject: Re: [PATCH v6] hashtable: introduce a small and naive hashtable

On Wed, 2012-09-26 at 10:39 -0400, Mathieu Desnoyers wrote:
> * Sasha Levin (levinsasha928@...il.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.

Coffee yes, but head cold as well. :-p

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

Bah, I was looking at the code the code and got the ret confused. I
originally had it the opposite, and then reversed it before sending.

> 
> 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 ?

It still works, as 'i' is not incremented due to the break. And i will
still be less than HASH_SIZE(hashtable). Did you have *your* cup of
coffee today? ;-)


> 
> 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.
> 

Nah,

-- Steve


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