On Sat, 23 Jul 2011, KOBAYASHI Yoshitake wrote:

(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

Good catch! Queued for -rt2.

Thanks,

	tglx