patches/0104-ipc-mqueue-Add-a-critical-section-to-avoid-a-deadloc.patch

From e207e326768d99ec27d1ee57338b4b6bc5e98584 Mon Sep 17 00:00:00 2001
From: KOBAYASHI Yoshitake <yoshitake.kobayashi@toshiba.co.jp>
Date: Sat, 23 Jul 2011 11:57:36 +0900
Subject: [PATCH 104/270] ipc/mqueue: Add a critical section to avoid a
 deadlock

(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 <yoshitake.kobayashi@toshiba.co.jp>
Cc: viro@zeniv.linux.org.uk
Cc: dchinner@redhat.com
Cc: npiggin@kernel.dk
Cc: hch@lst.de
Cc: arnd@arndb.de
Link: http://lkml.kernel.org/r/4E2A38A0.1090601@toshiba.co.jp
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 ipc/mqueue.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index dd9e66a..e630272 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -837,15 +837,19 @@ 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,
+ SYSCALL_DEFINE5(mq_timedsend, mqd_t, mqdes, const char __user *, u_msg_ptr,
 		size_t, msg_len, unsigned int, msg_prio,
 		const struct timespec __user *, u_abs_timeout)
 {
-- 
1.7.10.4