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    SubjectRe: Mysterious CFQ crash and RCU
    On Tue, May 24, 2011 at 12:20:40AM +0200, Paul Bolle wrote:
    > On Mon, 2011-05-23 at 08:38 -0700, Paul E. McKenney wrote:
    > > Running under CONFIG_PREEMPT=y (along with CONFIG_TREE_PREEMPT_RCU=y)
    > > could be very helpful in and of itself. CONFIG_DEBUG_OBJECTS_RCU_HEAD=y
    > > can also be helpful. In post-2.6.39 mainline, it should be possible
    > > to set CONFIG_DEBUG_OBJECTS_RCU_HEAD=y without CONFIG_PREEMPT=y, but
    > > again, CONFIG_PREEMPT=y can help find problems.
    >
    > 0) The first thing I tried (from your suggestions) was
    > CONFIG_DEBUG_OBJECTS_RCU_HEAD=y. Given its dependencies (and, well, the
    > build system I used) I ended up with:
    >
    > $ grep -e PREEMPT -e RCU /boot/config-2.6.39-0.local3.fc16.i686 |
    > grep -v "^#"
    > CONFIG_TREE_PREEMPT_RCU=y
    > CONFIG_PREEMPT_RCU=y
    > CONFIG_RCU_FANOUT=32
    > CONFIG_PREEMPT_NOTIFIERS=y
    > CONFIG_PREEMPT=y
    > CONFIG_DEBUG_OBJECTS_RCU_HEAD=y
    > CONFIG_DEBUG_PREEMPT=y
    > CONFIG_PROVE_RCU=y
    > CONFIG_SPARSE_RCU_POINTER=y
    >
    > It looks like I am unable to trigger the issue we're talking about here
    > when using that config.

    Interesting. One way for this to happen is to block inside an RCU
    read-side critical section. I need to think about appropriate
    diagnostics for this.

    > 1) For reference, the config of a kernel that does trigger it had:
    >
    > $ grep -e PREEMPT -e RCU /boot/config-2.6.39-0.local2.fc16.i686 |
    > grep -v "^#"
    > CONFIG_TREE_RCU=y
    > CONFIG_RCU_FANOUT=32
    > CONFIG_RCU_FAST_NO_HZ=y
    > CONFIG_PREEMPT_NOTIFIERS=y
    > CONFIG_PREEMPT_VOLUNTARY=y
    > CONFIG_PROVE_RCU=y
    > CONFIG_SPARSE_RCU_POINTER=y
    >
    > > > Again CONFIG_TREE_PREEMPT_RCU is available only if PREEMPT=y. So should
    > > > we enable preemtion and CONFIG_TREE_PREEMPT_RCU=y and try to reproduce
    > > > the issue?
    > >
    > > Please!
    >
    > 2) It appears I can't reproduce with those options enabled (see above).
    >
    > > Polling is fine. Please see attached for a script to poll at 15-second
    > > intervals. Please also feel free to adjust, just tell me what you
    > > adjusted.
    >
    > And should I now try to run that script on a config that triggers this
    > issue (such as the config under 1) above)?

    It might help while I am working out more targetted diagnostics.

    Thanx, Paul


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