From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx155.postini.com [74.125.245.155]) by kanga.kvack.org (Postfix) with SMTP id 069466B0032 for ; Tue, 3 Sep 2013 14:07:11 -0400 (EDT) Message-ID: <1378231630.17792.6.camel@buesod1.americas.hpqcorp.net> Subject: Re: [PATCH] ipc/msg.c: Fix lost wakeup in msgsnd(). From: Davidlohr Bueso Date: Tue, 03 Sep 2013 11:07:10 -0700 In-Reply-To: <1378216808-2564-1-git-send-email-manfred@colorfullife.com> References: <1378216808-2564-1-git-send-email-manfred@colorfullife.com> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Manfred Spraul Cc: Linus Torvalds , Andrew Morton , Davidlohr Bueso , Sedat Dilek , Davidlohr Bueso , linux-next , LKML , Stephen Rothwell , linux-mm , Andi Kleen , Rik van Riel , Jonathan Gonzalez , Vineet Gupta On Tue, 2013-09-03 at 16:00 +0200, Manfred Spraul wrote: > The check if the queue is full and adding current to the wait queue of pending > msgsnd() operations (ss_add()) must be atomic. > > Otherwise: > - the thread that performs msgsnd() finds a full queue and decides to sleep. > - the thread that performs msgrcv() calls first reads all messages from the > queue and then sleep, because the queue is empty. > - the msgrcv() calls do not perform any wakeups, because the msgsnd() task > has not yet called ss_add(). > - then the msgsnd()-thread first calls ss_add() and then sleeps. > Net result: msgsnd() and msgrcv() both sleep forever. > > Observed with msgctl08 from ltp with a preemptible kernel. Good catch, thanks for looking into this Manfred. FWIW similar changes that aim at reducing the kern_ipc_perm.lock contention in shm have already been in linux-next for a good while and should be going into 3.12. While both Sedat and I have tested them through LTP, I will keep an eye open for regressions so that we don't run into issues like this, late in the release cycle. > > Fix: Call ipc_lock_object() before performing the check. > > The patch also moves security_msg_queue_msgsnd() under ipc_lock_object: > - msgctl(IPC_SET) explicitely mentions that it tries to expunge any pending > operations that are not allowed anymore with the new permissions. > If security_msg_queue_msgsnd() is called without locks, then there might be > races. Right. > - it makes the patch much simpler. > > Reported-by: Vineet Gupta > Signed-off-by: Manfred Spraul Signed-off-by: Davidlohr Bueso > --- > ipc/msg.c | 12 +++++------- > 1 file changed, 5 insertions(+), 7 deletions(-) > > diff --git a/ipc/msg.c b/ipc/msg.c > index 9f29d9e..b65fdf1 100644 > --- a/ipc/msg.c > +++ b/ipc/msg.c > @@ -680,16 +680,18 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext, > goto out_unlock1; > } > > + ipc_lock_object(&msq->q_perm); > + > for (;;) { > struct msg_sender s; > > err = -EACCES; > if (ipcperms(ns, &msq->q_perm, S_IWUGO)) > - goto out_unlock1; > + goto out_unlock0; > > err = security_msg_queue_msgsnd(msq, msg, msgflg); > if (err) > - goto out_unlock1; > + goto out_unlock0; > > if (msgsz + msq->q_cbytes <= msq->q_qbytes && > 1 + msq->q_qnum <= msq->q_qbytes) { > @@ -699,10 +701,9 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext, > /* queue full, wait: */ > if (msgflg & IPC_NOWAIT) { > err = -EAGAIN; > - goto out_unlock1; > + goto out_unlock0; > } > > - ipc_lock_object(&msq->q_perm); > ss_add(msq, &s); > > if (!ipc_rcu_getref(msq)) { > @@ -730,10 +731,7 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext, > goto out_unlock0; > } > > - ipc_unlock_object(&msq->q_perm); > } > - > - ipc_lock_object(&msq->q_perm); > msq->q_lspid = task_tgid_vnr(current); > msq->q_stime = get_seconds(); > -- 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/ . Don't email: email@kvack.org