From patchwork Mon Dec 31 13:21:00 2012 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Eric Wong X-Patchwork-Id: 208856 X-Patchwork-Delegate: davem@davemloft.net Return-Path: X-Original-To: patchwork-incoming@ozlabs.org Delivered-To: patchwork-incoming@ozlabs.org Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by ozlabs.org (Postfix) with ESMTP id 8DBEB2C00A9 for ; Tue, 1 Jan 2013 00:21:21 +1100 (EST) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751234Ab2LaNVQ (ORCPT ); Mon, 31 Dec 2012 08:21:16 -0500 Received: from dcvr.yhbt.net ([64.71.152.64]:56466 "EHLO dcvr.yhbt.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751011Ab2LaNVO (ORCPT ); Mon, 31 Dec 2012 08:21:14 -0500 Received: from localhost (dcvr.yhbt.net [127.0.0.1]) by dcvr.yhbt.net (Postfix) with ESMTP id 19FF41F433; Mon, 31 Dec 2012 13:21:14 +0000 (UTC) From: Eric Wong To: linux-kernel@vger.kernel.org Cc: Eric Wong , Hans Verkuil , Jiri Olsa , Jonathan Corbet , Al Viro , Davide Libenzi , Hans de Goede , Mauro Carvalho Chehab , David Miller , Eric Dumazet , Andrew Morton , Linus Torvalds , Andreas Voellmy , "Junchang(Jason) Wang" , netdev@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: [PATCH] poll: prevent missed events if _qproc is NULL Date: Mon, 31 Dec 2012 13:21:00 +0000 Message-Id: <1356960060-1263-1-git-send-email-normalperson@yhbt.net> X-Mailer: git-send-email 1.8.1.rc2.8.ga09f24b In-Reply-To: <20121228014503.GA5017@dcvr.yhbt.net> References: <20121228014503.GA5017@dcvr.yhbt.net> Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org This patch seems to fix my issue with ppoll() being stuck on my SMP machine: http://article.gmane.org/gmane.linux.file-systems/70414 The change to sock_poll_wait() in commit 626cf236608505d376e4799adb4f7eb00a8594af (poll: add poll_requested_events() and poll_does_not_wait() functions) seems to have allowed additional cases where the SMP memory barrier is not issued before checking for readiness. In my case, this affects the select()-family of functions which register descriptors once and set _qproc to NULL before checking events again (after poll_schedule_timeout() returns). The set_mb() barrier in poll_schedule_timeout() appears to be insufficient on my SMP x86-64 machine (as it's only an xchg()). This may also be related to the epoll issue described by Andreas Voellmy in http://thread.gmane.org/gmane.linux.kernel/1408782/ Signed-off-by: Eric Wong Cc: Hans Verkuil Cc: Jiri Olsa Cc: Jonathan Corbet Cc: Al Viro Cc: Davide Libenzi Cc: Hans de Goede Cc: Mauro Carvalho Chehab Cc: David Miller Cc: Eric Dumazet Cc: Andrew Morton Cc: Linus Torvalds Cc: Andreas Voellmy Cc: "Junchang(Jason) Wang" Cc: netdev@vger.kernel.org Cc: linux-fsdevel@vger.kernel.org --- If this patch is correct, I think we can just drop the poll_does_not_wait() function entirely since poll_wait() does the same check anyways... include/net/sock.h | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/include/net/sock.h b/include/net/sock.h index c945fba..1923e48 100644 --- a/include/net/sock.h +++ b/include/net/sock.h @@ -1925,8 +1925,9 @@ static inline bool wq_has_sleeper(struct socket_wq *wq) static inline void sock_poll_wait(struct file *filp, wait_queue_head_t *wait_address, poll_table *p) { - if (!poll_does_not_wait(p) && wait_address) { - poll_wait(filp, wait_address, p); + if (wait_address) { + if (!poll_does_not_wait(p)) + poll_wait(filp, wait_address, p); /* We need to be sure we are in sync with the * socket flags modification. *