From mboxrd@z Thu Jan 1 00:00:00 1970 From: KOBAYASHI Yoshitake Subject: [PATCH 3.0-rt1] ipc/mqueue: add a critical section to avoid a deadlock Date: Sat, 23 Jul 2011 11:57:36 +0900 Message-ID: <4E2A38A0.1090601@toshiba.co.jp> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-2022-JP Content-Transfer-Encoding: 7bit Cc: linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org, viro@zeniv.linux.org.uk, dchinner@redhat.com, npiggin@kernel.dk, hch@lst.de, arnd@arndb.de To: tglx@linutronix.de Return-path: Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-rt-users.vger.kernel.org (Repost for v3.0-rt1 and changed the distination addreses) I have tested the following patch on v3.0-rt1 with PREEMPT_RT_FULL. In POSIX message queue, if a sender process uses SCHED_FIFO and has a higher priority than a receiver process, the sender will be stuck at ipc/mqueue.c:452 452 while (ewp->state == STATE_PENDING) 453 cpu_relax(); Description of the problem (receiver process) 1. receiver changes sender's state to STATE_PENDING (mqueue.c:846) 2. wake up sender process and "switch to sender" (mqueue.c:847) Note: This context switch only happens in PREEMPT_RT_FULL kernel. (sender process) 3. sender check the own state in above loop (mqueue.c:452-453) *. receiver will never wake up and cannot change sender's state to STATE_READY because sender has higher priority Signed-off-by: Yoshitake Kobayashi --- ipc/mqueue.c | 5 +++++ 1 files changed, 5 insertions(+), 0 deletions(-) diff --git a/ipc/mqueue.c b/ipc/mqueue.c index 22a06a4..1cf6e53 100644 --- a/ipc/mqueue.c +++ b/ipc/mqueue.c @@ -841,12 +841,17 @@ static inline void pipelined_receive(struct mqueue_inode_info *info) wake_up_interruptible(&info->wait_q); return; } + /* + * Keep them in one critical section for PREEMPT_RT: + */ + preempt_disable_rt(); msg_insert(sender->msg, info); list_del(&sender->list); sender->state = STATE_PENDING; wake_up_process(sender->task); smp_wmb(); sender->state = STATE_READY; + preempt_enable_rt(); } SYSCALL_DEFINE5(mq_timedsend, mqd_t, mqdes, const char __user *, u_msg_ptr, -- 1.7.2.5