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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Wed, 26 Sep 2012 10:39:05 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	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

* 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.

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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/