Messages in this thread | | | Date | Tue, 17 Jan 2006 22:53:04 -0800 | From | Andrew Morton <> | Subject | Re: 2.6.15-mm4 failure on power5 |
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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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