From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail144.messagelabs.com (mail144.messagelabs.com [216.82.254.51]) by kanga.kvack.org (Postfix) with ESMTP id 230C26B016A for ; Mon, 22 Aug 2011 16:52:24 -0400 (EDT) Date: Mon, 22 Aug 2011 13:52:18 -0700 From: Andrew Morton Subject: Re: [PATCH 3/4] string: introduce memchr_inv Message-Id: <20110822135218.f2d9f462.akpm@linux-foundation.org> In-Reply-To: <1314030548-21082-4-git-send-email-akinobu.mita@gmail.com> References: <1314030548-21082-1-git-send-email-akinobu.mita@gmail.com> <1314030548-21082-4-git-send-email-akinobu.mita@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Akinobu Mita Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Christoph Lameter , Pekka Enberg , Matt Mackall , Joern Engel , logfs@logfs.org, Marcin Slusarz , Eric Dumazet , linux-arch@vger.kernel.org On Tue, 23 Aug 2011 01:29:07 +0900 Akinobu Mita wrote: > memchr_inv() is mainly used to check whether the whole buffer is filled > with just a specified byte. > > The function name and prototype are stolen from logfs and the > implementation is from SLUB. > > ... > > +/** > + * memchr_inv - Find a character in an area of memory. > + * @s: The memory area > + * @c: The byte to search for > + * @n: The size of the area. This text seems to be stolen from memchr(). I guess it's close enough. > + * returns the address of the first character other than @c, or %NULL > + * if the whole buffer contains just @c. > + */ > +void *memchr_inv(const void *start, int c, size_t bytes) > +{ > + u8 value = c; > + u64 value64; > + unsigned int words, prefix; > + > + if (bytes <= 16) > + return check_bytes8(start, value, bytes); > + > + value64 = value | value << 8 | value << 16 | value << 24; > + value64 = (value64 & 0xffffffff) | value64 << 32; > + prefix = 8 - ((unsigned long)start) % 8; > + > + if (prefix) { > + u8 *r = check_bytes8(start, value, prefix); > + if (r) > + return r; > + start += prefix; > + bytes -= prefix; > + } > + > + words = bytes / 8; > + > + while (words) { > + if (*(u64 *)start != value64) OK, problem. This will explode if passed a misaligned address on certain (non-x86) architectures. This is nasty because people will develop and test code on x86 and it works. Much later, the alpha/ia64/etc guys discover the problem. One fix would be to use get_unaligned(). This might be slow on some architectures, I don't know. Another fix is to restrict the caller's alignment freedom; document this and add a runtime WARN_ON(). > + return check_bytes8(start, value, 8); > + start += 8; > + words--; > + } > + > + return check_bytes8(start, value, bytes % 8); > +} > +EXPORT_SYMBOL(memchr_inv); -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org