Subject: ipc/mqueue: Add a critical section to avoid a deadlock From: KOBAYASHI Yoshitake Date: Sat, 23 Jul 2011 11:57:36 +0900 Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/3.10/patches-3.10.4-rt1.tar.xz (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 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 --- 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 @@ -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,