Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760918Ab2J2U7i (ORCPT ); Mon, 29 Oct 2012 16:59:38 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:60718 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757923Ab2J2U7h (ORCPT ); Mon, 29 Oct 2012 16:59:37 -0400 Date: Mon, 29 Oct 2012 13:59:35 -0700 From: Andrew Morton To: Yuanhan Liu Cc: linux-kernel@vger.kernel.org, Stefani Seibold Subject: Re: [PATCH 1/2] kfifo: round up the fifo size power of 2 Message-Id: <20121029135935.bb8b0b2a.akpm@linux-foundation.org> In-Reply-To: <1351238218-22648-1-git-send-email-yuanhan.liu@linux.intel.com> References: <1351238218-22648-1-git-send-email-yuanhan.liu@linux.intel.com> X-Mailer: Sylpheed 3.0.2 (GTK+ 2.20.1; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1789 Lines: 50 On Fri, 26 Oct 2012 15:56:57 +0800 Yuanhan Liu wrote: > Say, if we want to allocate a filo with size of 6 bytes, it would be safer > to allocate 8 bytes instead of 4 bytes. > > ... > > --- a/kernel/kfifo.c > +++ b/kernel/kfifo.c > @@ -39,11 +39,11 @@ int __kfifo_alloc(struct __kfifo *fifo, unsigned int size, > size_t esize, gfp_t gfp_mask) > { > /* > - * round down to the next power of 2, since our 'let the indices > + * round up to the next power of 2, since our 'let the indices > * wrap' technique works only in this case. > */ > if (!is_power_of_2(size)) > - size = rounddown_pow_of_two(size); > + size = roundup_pow_of_two(size); > > fifo->in = 0; > fifo->out = 0; > @@ -84,7 +84,7 @@ int __kfifo_init(struct __kfifo *fifo, void *buffer, > size /= esize; > > if (!is_power_of_2(size)) > - size = rounddown_pow_of_two(size); > + size = roundup_pow_of_two(size); > > fifo->in = 0; > fifo->out = 0; hm, well, if the user asked for a 100-element fifo then it is a bit strange and unexpected to give them a 128-element one. If there's absolutely no prospect that the kfifo code will ever support 100-byte fifos then I guess we should rework the API so that the caller has to pass in log2 of the size, not the size itself. That way there will be no surprises and no mistakes. That being said, the power-of-2 limitation isn't at all intrinsic to a fifo, so we shouldn't do this. Ideally, we'd change the kfifo implementation so it does what the caller asked it to do! -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/