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

From: Thomas Gleixner
Date: Sat Jul 23 2011 - 03:23:50 EST

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.


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