Messages in this thread | | | Date | Tue, 9 Jun 2009 13:46:50 -0700 | From | Gary Hade <> | Subject | Re: [RESEND] [PATCH v2] [BUGFIX] x86/x86_64: fix CPU offlining triggered "inactive" device IRQ interrruption |
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On Mon, Jun 08, 2009 at 12:19:30PM -0700, Eric W. Biederman wrote: > Ingo Molnar <mingo@elte.hu> writes: > > > * Gary Hade <garyhade@us.ibm.com> wrote: > > > >> On Wed, Jun 03, 2009 at 04:55:26AM -0700, Eric W. Biederman wrote: > >> > Gary Hade <garyhade@us.ibm.com> writes: > >> > > >> > > Impact: Eliminates a race that can leave the system in an > >> > > unusable state > >> > > > >> > > During rapid offlining of multiple CPUs there is a chance > >> > > that an IRQ affinity move destination CPU will be offlined > >> > > before the IRQ affinity move initiated during the offlining > >> > > of a previous CPU completes. This can happen when the device > >> > > is not very active and thus fails to generate the IRQ that is > >> > > needed to complete the IRQ affinity move before the move > >> > > destination CPU is offlined. When this happens there is an > >> > > -EBUSY return from __assign_irq_vector() during the offlining > >> > > of the IRQ move destination CPU which prevents initiation of > >> > > a new IRQ affinity move operation to an online CPU. This > >> > > leaves the IRQ affinity set to an offlined CPU. > >> > > > >> > > I have been able to reproduce the problem on some of our > >> > > systems using the following script. When the system is idle > >> > > the problem often reproduces during the first CPU offlining > >> > > sequence. > >> > > >> > Nacked-by: "Eric W. Biederman" <ebiederm@xmission.com> > >> > > >> > fixup_irqs() is broken for allowing such a thing. > >> > >> When fixup_irqs() calls the set_affinity function: > >> ... > >> if (desc->chip->set_affinity) > >> desc->chip->set_affinity(irq, affinity); > >> ... > >> it receives no feedback so it obviously expects the set_affinity > >> function or it's called functions to do the right thing by preventing > >> or correctly handling any problems that should arise. In the case of > >> this bug there is obviously a problem happening during the set_affinity > >> function call that needs to be resolved and/or properly handled. > >> > >> When you made your "x86_64 irq: Safely cleanup an irq after moving it." > >> changes (re: http://lkml.org/lkml/2007/2/23/92) you added the check > >> to __assign_irq_vector() that causes it to return -EBUSY if the > >> migration of the IRQ is still in progress: > >> + if ((cfg->move_in_progress) || cfg->move_cleanup_count) > >> + return -EBUSY; > >> + > >> However, you did not add any code to other functions on the > >> call stack to properly deal with this error. When doing this > >> you may have assumed (as I may have also assumed) that the underlying > >> code was solid enough that the handling was not needed. Unfortunately, > >> you apparently did not anticipate the case where an idle or relatively > >> idle device may not generate the IRQ needed to complete the move > >> before the CPU that is still handling that IRQ is offlined. > >> > >> My fix only addresses the issue that caused the -EBUSY return > >> and subsequent mess. It does not address the omitted handling > >> for this error condition. If we were to add the handling to > >> fixup_irq() and the arch and non-arch specific functions above > >> it on the call stack as you may be suggesting, it would be quite > >> involved because of all the things that would need to be undone. > >> > >> I am not certain that my fix plugs the very last hole that could > >> cause the -EBUSY return from __assign_irq_vector() so maybe we > >> should at least add a warning or BUG_ON to make the unhandled > >> error more obvious in the future. I would be happy to provide > >> this via a separate patch. > > > > A WARN_ON_ONCE() patch would certainly be nice, as a reminder and as > > a prodder-tool. > > In fixup_irqs such a warning would be reasonable. In assign_irq_vector > it makes no sense. > > I just read through the code. Anything that assumes assign_irq_vector > will always succeed is BROKEN. We can not guarantee it. There are > also memory allocation failures and the fundamental problem that we > may have more irqs than can fit on a single cpu.
Yes, I am certain that there are other bugs lurking that haven't yet manifested into real and serious failures. I am simply proposing that we fix one very serious bug that has.
> > Furthermore while we require at least two irqs to complete a irq migration > I don't believe we can avoid returning -EBUSY there.
I didn't see anything in send_cleanup_vector() such as a memory allocation failure (already handled there) that, in the absense of a yet to be discoverd bug, should prevent cfg->move_in_progress from getting zeroed. Assuming a memory allocation failure in send_cleanup_vector() that brings cfg->move_cleanup_count into the picture, I didn't see anything, in the absense of a yet to be discovered bug, in smp_irq_move_cleanup_interrupt() that I thought would prevent cfg->move_cleanup_count from getting decremented to zero.
If you are still uncomfortable with this, the WARN_ON_ONCE could be limited to the instances where the CPU is being offlined i.e. the case that is known to be so very destructive.
Gary
-- Gary Hade System x Enablement IBM Linux Technology Center 503-578-4503 IBM T/L: 775-4503 garyhade@us.ibm.com http://www.ibm.com/linux/ltc
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