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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(-)
Index: linux-stable/ipc/mqueue.c
===================================================================
--- linux-stable.orig/ipc/mqueue.c
+++ linux-stable/ipc/mqueue.c
@@ -936,13 +936,18 @@ 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,
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