[PATCH 3.0-rt1] ipc/mqueue: add a critical section to avoid a deadlock

From: KOBAYASHI Yoshitake
Date: Fri Jul 22 2011 - 22:58:30 EST

(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@xxxxxxxxxxxxx>
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)
+ /*
+ * Keep them in one critical section for PREEMPT_RT:
+ */
+ preempt_disable_rt();
msg_insert(sender->msg, info);
sender->state = STATE_PENDING;
sender->state = STATE_READY;
+ preempt_enable_rt();

SYSCALL_DEFINE5(mq_timedsend, mqd_t, mqdes, const char __user *, u_msg_ptr,
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/