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    SubjectRe: 2.6.31.4-rt14 posix message queues problem


    On Thu, 15 Oct 2009, John Kacur wrote:

    >
    > > From: Joerg Abraham <Joerg.Abraham@alcatel-lucent.de>
    > > Date: Thu, Oct 15, 2009 at 7:51 PM
    > > Subject: 2.6.31.4-rt14 posix message queues problem
    > > To: LKML <linux-kernel@vger.kernel.org>
    > >
    > >
    > > Hi,
    > >
    > > During some performance measurement I stumbled over unexpected bad
    > > results for a posix message queue test case. Which is not surprising
    > > since the kernel log shows a "BUG: scheduling while atomic:" on every
    > > measurement trigger.
    > >
    > > Attached is the kernel log for an embedded 8572ds powerpc system and a
    > > condensed user space test appli. The test appli uses 2 processes
    > > (mq_server and mq_client) and pingpongs some messages via posix
    > > message queues. Please see the c-file header for details (the appli
    > > itself makes not much sense but shows the problem).
    > >
    > > ./mq_server
    > > ./mq_client # just crashes during run
    > >
    > > The problem does _not_ occur on kernels 2.6.29.6-rt24, 2.6.29.6
    > > vanilla and 2.6.31.4 vanilla. The problem does _also_ show up on an
    > > embedded x86_64 system and a full featured fc11 x86_64 system running
    > > a 2.6.31.4-rt14 based kernel, so I dont think it's arch dependent.
    > >
    > > Can anybody reproduce the crash ???
    > >
    > > If you need more info's please let me know.
    > >
    > > [And if I'm doing stupid things just forgive me]
    > >
    > > Thanx
    > >
    > > Joerg
    >
    > Hello Joerg
    >
    > Does the following patch solve your problem?
    >
    > From 69fde4357db4a054733cbca4158606160e821a3a Mon Sep 17 00:00:00 2001
    > From: John Kacur <jkacur@redhat.com>
    > Date: Thu, 15 Oct 2009 20:52:37 +0200
    > Subject: [PATCH] preempt_disable_rt(); should be paired with preempt_enable_rt()
    >
    > Signed-off-by: John Kacur <jkacur@redhat.com>
    > ---
    > ipc/mqueue.c | 2 +-
    > 1 files changed, 1 insertions(+), 1 deletions(-)
    >
    > diff --git a/ipc/mqueue.c b/ipc/mqueue.c
    > index 63a47f7..ab4df36 100644
    > --- a/ipc/mqueue.c
    > +++ b/ipc/mqueue.c
    > @@ -830,7 +830,7 @@ static inline void pipelined_send(struct mqueue_inode_info *info,
    > wake_up_process(receiver->task);
    > smp_wmb();
    > receiver->state = STATE_READY;
    > - preempt_enable_nort();
    > + preempt_enable_rt();
    > }
    >
    > /* pipelined_receive() - if there is task waiting in sys_mq_timedsend()
    > --
    > 1.6.0.6
    >
    >

    fyi: I completed my testing, and I was able to reproduce your problem on
    v2.6.31.4-rt14, and it goes away with the above patch.

    Cheers

    John


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