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

From: Mathieu Desnoyers
Date: Wed Sep 26 2012 - 12:19:52 EST


* Steven Rostedt (rostedt@xxxxxxxxxxx) wrote:
> On Wed, 2012-09-26 at 10:39 -0400, Mathieu Desnoyers wrote:
> > * Sasha Levin (levinsasha928@xxxxxxxxx) 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? ;-)

Ahh, right! Actually I had it already ;-)

>
>
> >
> > 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,

Agreed that the flags should be removed. Moving to define + static
inline is still important though.

Thanks,

Mathieu


>
> -- Steve
>
>

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