Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757944AbaAJStu (ORCPT ); Fri, 10 Jan 2014 13:49:50 -0500 Received: from g4t0014.houston.hp.com ([15.201.24.17]:46769 "EHLO g4t0014.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754146AbaAJSsr (ORCPT ); Fri, 10 Jan 2014 13:48:47 -0500 From: Davidlohr Bueso To: akpm@linux-foundation.org, manfred@colorfullife.com Cc: riel@redhat.com, aswin@hp.com, davidlohr@hp.com, linux-kernel@vger.kernel.org Subject: [PATCH 7/7] ipc,msg: document barriers Date: Fri, 10 Jan 2014 10:48:27 -0800 Message-Id: <1389379707-20298-8-git-send-email-davidlohr@hp.com> X-Mailer: git-send-email 1.8.1.4 In-Reply-To: <1389379707-20298-1-git-send-email-davidlohr@hp.com> References: <1389379707-20298-1-git-send-email-davidlohr@hp.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Both expunge_all() and pipeline_send() rely on both a nil msg value and a full barrier to guarantee the correct ordering when waking up a task. While its counter part at the receiving end is well documented for the lockless recv algorithm, we still need to document these specific smp_mb() calls. Signed-off-by: Davidlohr Bueso --- ipc/msg.c | 19 +++++++++++++++++-- 1 file changed, 17 insertions(+), 2 deletions(-) diff --git a/ipc/msg.c b/ipc/msg.c index 4377f4a..8c1cc76 100644 --- a/ipc/msg.c +++ b/ipc/msg.c @@ -253,8 +253,14 @@ static void expunge_all(struct msg_queue *msq, int res) struct msg_receiver *msr, *t; list_for_each_entry_safe(msr, t, &msq->q_receivers, r_list) { - msr->r_msg = NULL; + msr->r_msg = NULL; /* initialize expunge ordering */ wake_up_process(msr->r_tsk); + /* + * Ensure that the wakeup is visible before setting r_msg as + * the receiving end depends on it: either spinning on a nil, + * or dealing with -EAGAIN cases. See lockless reveice part 1 + * and 2 in do_msgrcv(). + */ smp_mb(); msr->r_msg = ERR_PTR(res); } @@ -638,15 +644,22 @@ static inline int pipelined_send(struct msg_queue *msq, struct msg_msg *msg) list_del(&msr->r_list); if (msr->r_maxsize < msg->m_ts) { + /* initialize pipelined send ordering */ msr->r_msg = NULL; wake_up_process(msr->r_tsk); - smp_mb(); + smp_mb(); /* see barrier comment below */ msr->r_msg = ERR_PTR(-E2BIG); } else { msr->r_msg = NULL; msq->q_lrpid = task_pid_vnr(msr->r_tsk); msq->q_rtime = get_seconds(); wake_up_process(msr->r_tsk); + /* + * Ensure that the wakeup is visible before + * setting r_msg, as the receiving end depends + * on it. See lockless reveice part 1 and 2 in + * do_msgrcv(). + */ smp_mb(); msr->r_msg = msg; @@ -654,6 +667,7 @@ static inline int pipelined_send(struct msg_queue *msq, struct msg_msg *msg) } } } + return 0; } @@ -716,6 +730,7 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext, goto out_unlock0; } + /* enqueue the sender and prepare to block */ ss_add(msq, &s); if (!ipc_rcu_getref(msq)) { -- 1.8.1.4 -- 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/