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    SubjectRe: 2.6.15-mm4 failure on power5
    Ingo Molnar <mingo@elte.hu> wrote:
    >
    >
    > * Dave C Boutcher <sleddog@us.ibm.com> wrote:
    >
    > > On Wed, Jan 18, 2006 at 11:19:36AM +1100, Michael Ellerman wrote:
    > > > It booted fine _with_ the patch applied, with DEBUG_MUTEXES=y and n.
    > > >
    > > > Boutcher, to be clear, you can't boot with kernel-kernel-cpuc-to-mutexes.patch
    > > > applied and DEBUG_MUTEXES=y ?
    > > >
    > > > But if you revert kernel-kernel-cpuc-to-mutexes.patch it boots ok?
    > > >
    > > > This is looking quite similar to another hang we're seeing on Power4 iSeries
    > > > on mainline git:
    > > > http://ozlabs.org/pipermail/linuxppc64-dev/2006-January/007679.html
    > >
    > > Correct...I die in exactly the same place every time with
    > > DEBUG_MUTEXES=Y. I posted a backtrace that points into the _lock_cpu
    > > code, but I haven't really dug into the issue yet. I believe this is
    > > very timing related (Serge was dying slightly differently).
    >
    > so my question still is: _without_ the workaround patch, i.e. with
    > vanilla -mm4, and DEBUG_MUTEXES=n, do you get a hang?
    >
    > the reason for my question is that DEBUG_MUTEXES=y will e.g. enable
    > interrupts

    That used to kill ppc64 and yes, it died in timer interrupts.

    > - so buggy early bootup code which relies on interrupts being
    > off might be surprised by it.

    I don't think it's necessarily buggy that bootup code needs interrupts
    disabled. It _is_ buggy that bootup code which needs interrupts disabled
    is calling lock_cpu_hotplug().

    > The fact that you observed that it's
    > somehow related to the timer interrupt seems to strengthen this
    > suspicion. DEBUG_MUTEXES=n on the other hand should have no such
    > interrupt-enabling effects.
    >
    > [ if this indeed is the case then i'll add irqs_off() checks to
    > DEBUG_MUTEXES=y, to ensure that the mutex APIs are never called with
    > interrupts disabled. ]

    Yes, I suppose so. But we're already calling might_sleep(), and
    might_sleep() checks for that. Perhaps the might_sleep() check is being
    defeated by the nasty system_running check.

    There's a sad story behind that system_running check in might_sleep().
    Because the kernel early boot is running in an in_atomic() state, a great
    number of bogus might_sleep() warnings come out because of various code
    doing potentially-sleepy things. I ended up adding the system_running
    test, with the changelog "OK, I give up. Kill all the might_sleep warnings
    from the early boot process." Undoing that and fixing up the fallout would
    be a lot of nasty work.


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