From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756334AbcHVPqa (ORCPT ); Mon, 22 Aug 2016 11:46:30 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:26903 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1755738AbcHVPp3 (ORCPT ); Mon, 22 Aug 2016 11:45:29 -0400 X-IBM-Helo: d03dlp03.boulder.ibm.com X-IBM-MailFrom: paulmck@linux.vnet.ibm.com From: "Paul E. McKenney" To: linux-kernel@vger.kernel.org Cc: mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com, Chris Wilson , "Paul E. McKenney" , Dan Williams , Jan Kara , Josef Bacik Subject: [PATCH tip/core/rcu 1/7] list: Expand list_first_entry_or_null() Date: Mon, 22 Aug 2016 08:45:24 -0700 X-Mailer: git-send-email 2.5.2 In-Reply-To: <20160822154501.GA10364@linux.vnet.ibm.com> References: <20160822154501.GA10364@linux.vnet.ibm.com> X-TM-AS-GCONF: 00 X-Content-Scanned: Fidelis XPS MAILER x-cbid: 16082215-8235-0000-0000-00000908B87B X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00005629; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000183; SDB=6.00748123; UDB=6.00353038; IPR=6.00520789; BA=6.00004672; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00012420; XFM=3.00000011; UTC=2016-08-22 15:45:25 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 16082215-8236-0000-0000-0000342ACB8F Message-Id: <1471880730-11180-1-git-send-email-paulmck@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2016-08-22_07:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=1 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1604210000 definitions=main-1608220164 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Chris Wilson Due to the use of READ_ONCE() in list_empty() the compiler cannot optimise !list_empty() ? list_first_entry() : NULL very well. By manually expanding list_first_entry_or_null() we can take advantage of the READ_ONCE() to avoid the list element changing under the test while the compiler can generate smaller code. Signed-off-by: Chris Wilson Cc: "Paul E. McKenney" Cc: Andrew Morton Cc: Dan Williams Cc: Jan Kara Cc: Josef Bacik Cc: linux-kernel@vger.kernel.org Signed-off-by: Paul E. McKenney --- include/linux/list.h | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/include/linux/list.h b/include/linux/list.h index 5183138aa932..5809e9a2de5b 100644 --- a/include/linux/list.h +++ b/include/linux/list.h @@ -381,8 +381,11 @@ static inline void list_splice_tail_init(struct list_head *list, * * Note that if the list is empty, it returns NULL. */ -#define list_first_entry_or_null(ptr, type, member) \ - (!list_empty(ptr) ? list_first_entry(ptr, type, member) : NULL) +#define list_first_entry_or_null(ptr, type, member) ({ \ + struct list_head *head__ = (ptr); \ + struct list_head *pos__ = READ_ONCE(head__->next); \ + pos__ != head__ ? list_entry(pos__, type, member) : NULL; \ +}) /** * list_next_entry - get the next element in list -- 2.5.2