patches/ipc-mqueue-add-a-critical-section-to-avoid-a-deadlock.patch

Subject: ipc/mqueue: Add a critical section to avoid a deadlock
From: KOBAYASHI Yoshitake <yoshitake.kobayashi@toshiba.co.jp>
Date: Sat, 23 Jul 2011 11:57:36 +0900

(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 |   19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -945,13 +945,18 @@ static inline void pipelined_receive(str
 		wake_up_interruptible(&info->wait_q);
 		return;
 	}
-	if (msg_insert(sender->msg, info))
-		return;
-	list_del(&sender->list);
-	sender->state = STATE_PENDING;
-	wake_up_process(sender->task);
-	smp_wmb();
-	sender->state = STATE_READY;
+	/*
+	 * Keep them in one critical section for PREEMPT_RT:
+	 */
+	preempt_disable_rt();
+	if (!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,