From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx123.postini.com [74.125.245.123]) by kanga.kvack.org (Postfix) with SMTP id D59726B0099 for ; Thu, 2 Aug 2012 16:42:04 -0400 (EDT) Date: Thu, 2 Aug 2012 13:41:57 -0700 From: Josh Triplett Subject: Re: [RFC 1/4] hashtable: introduce a small and naive hashtable Message-ID: <20120802204157.GB7916@jtriplet-mobl1> References: <5019B0B4.1090102@gmail.com> <20120801224556.GF15477@google.com> <501A4FC1.8040907@gmail.com> <20120802103244.GA23318@leaf> <501A633B.3010509@gmail.com> <501A7AD3.7000008@gmail.com> <20120802161556.GA25572@leaf> <501AAF47.3090708@gmail.com> <20120802174457.GA6251@jtriplet-mobl1> <501ABEE2.10007@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <501ABEE2.10007@gmail.com> Sender: owner-linux-mm@kvack.org List-ID: To: Sasha Levin Cc: Tejun Heo , torvalds@linux-foundation.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, paul.gortmaker@windriver.com On Thu, Aug 02, 2012 at 07:54:42PM +0200, Sasha Levin wrote: > On 08/02/2012 07:44 PM, Josh Triplett wrote: > > On Thu, Aug 02, 2012 at 06:48:07PM +0200, Sasha Levin wrote: > >> On 08/02/2012 06:15 PM, Josh Triplett wrote: > >>> On Thu, Aug 02, 2012 at 03:04:19PM +0200, Sasha Levin wrote: > >>>> On 08/02/2012 01:23 PM, Sasha Levin wrote: > >>>>>> #define DEFINE_HASH_TABLE(name, length) struct hash_table name = { .count = length, .buckets = { [0 ... (length - 1)] = HLIST_HEAD_INIT } } > >>>>> The limitation of this approach is that the struct hash_table variable must be 'static', which is a bit limiting - see for example the use of hashtable in 'struct user_namespace'. > >>>>> > >>>> > >>>> What if we just use two possible decelerations? One of static structs and one for regular ones. > >>>> > >>>> struct hash_table { > >>>> size_t bits; > >>>> struct hlist_head buckets[]; > >>>> }; > >>>> > >>>> #define DEFINE_HASHTABLE(name, bits) \ > >>>> union { \ > >>>> struct hash_table name; \ > >>>> struct { \ > >>>> size_t bits; \ > >>> > >>> This shouldn't use "bits", since it'll get expanded to the macro > >>> argument. > >>> > >>>> struct hlist_head buckets[1 << bits]; \ > >>>> } __name; \ > >>> > >>> __##name > >>> > >>>> } > >>>> > >>>> #define DEFINE_STATIC_HASHTABLE(name, bit) \ > >>>> static struct hash_table name = { .bits = bit, \ > >>>> .buckets = { [0 ... (bit - 1)] = HLIST_HEAD_INIT } } > >>> > >>> You probably wanted to change that to [0 ... ((1 << bit) - 1)] , to > >>> match DEFINE_HASHTABLE. > >> > >> I wrote it by hand and didn't compile test, will fix all of those. > >> > >>> Since your definition of DEFINE_HASHTABLE would also work fine when used > >>> statically, why not just always use that? > >>> > >>> #define DEFINE_STATIC_HASHTABLE(name, bits) static DEFINE_HASHTABLE(name, bits) = { .name.bits = bits } > >> > >> It will get defined fine, but it will be awkward to use. We'd need to pass anonymous union to all the functions that handle this hashtable, which isn't pretty. > > > > No, it'll still use the anonymous union, so you'll still have a thing of > > type "struct hash_table" with the given name, and you can use that name > > with the hash-table functions. > > We can use 'struct hash_table' directly, but then the call will look awkward :) > > Consider this case (I've placed arbitrary values into size and name: > > /* I've "preprocessed" the DEFINE macro below */ > union { > struct hash_table table; > struct { > size_t bits; > struct hlist_head buckets[32]; > } > } my_hashtable; That expansion doesn't match the macros. Using the most recent definitions of DEFINE_HASHTABLE and DEFINE_STATIC_HASHTABLE from above, the definition would look something like this: static union { struct hash_table my_hashtable; struct { size_t bits; struct hlist_head buckets[1 << 5]; } __my_hashtable; } = { .my_hashtable.bits = 5 }; > void foo(struct hash_table *table) > { > /* Do something */ > } > > int main(void) > { > foo(my_hashtable); /* This is what the user expects to work, and won't work in this case */ > > foo(&my_hashtable.table); /* This is what he has to do, which means the user has to know about the internal structure of the union */ > } Given the expansion above, you can just write this as foo(&my_hashtable), which seems sensible to me. - Josh Triplett -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org