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Subject[PATCH RT] ipc: add a critical section in pipelined_receive
This patch is for v2.6.33.9-rt31 to avoid a deadlock.
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:446

445 while (ewp->state == STATE_PENDING)
446 cpu_relax();

Description of the problem
(receiver process)
1. receiver changes sender's state to STATE_PENDING (mqueue.c:832)
2. wake up sender process and "switch to sender" (mqueue.c:833)
Note: This context switch only happens in PREEMPT_RT kernel.
(sender process)
3. sender check the own state in above loop (mqueue.c:445-446)
*. 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>
---
ipc/mqueue.c | 5 +++++
1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 01582e5..712d813 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -827,12 +827,17 @@ 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

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