Re: Bisected KVM hang on x86-32 between v3.12 and v3.13

From: Toralf FÃrster
Date: Sun Apr 06 2014 - 11:53:33 EST

Hash: SHA256

On 04/06/2014 05:19 PM, Michele Ballabio wrote:
> Toralf FÃrster reported this in
> "The issue happens here at a 32 bit stable Gentoo Linux if
> I try to start a KVM image. Kernels 3.12.X works fine,
> kernel >= v3.13 will hang shortly after I started the image
> with the virtual-manager. The last syslog messages are
> something like:
> Feb 28 16:22:00 n22 kernel: INFO: rcu_sched detected stalls
> on CPUs/tasks: {} (detected by 2, t=60002 jiffies,
> g=14689, c=14688, q=21051)
> Feb 28 16:22:00 n22 kernel: INFO: Stall ended before state
> dump start"
> He correctly pointed out that the bisection blamed the merge
> commit 37bf06375c90a42fe07b9bebdb07bc316ae5a0ce
> "Merge tag 'v3.12-rc4' into sched/core".
> This bug is obviously caused by at least two patches, one
> on each side of the merge, that only when combined together
> (at that merge point) cause the bug in kvm. By rebasing
> the "sched/core" branch on "master" before the merge and
> going on with the bisection, I found commit
> 3e8e42c69bb7d9fc12ebc23ff308e8523a2a59a0
> "sched: Revert need_resched() to look at TIF_NEED_RESCHED"
> as one of the causes. The other patch that contributes to the
> bug is commit ded797547548a5b8e7b92383a41e4c0e6b0ecb7f
> "irq: Force hardirq exit's softirq processing on its own stack".
> Reverting either one of them solves the problem reported with kvm,
> but revert is probably not the correct answer.
> I wonder if the solution is as simple as this:
> --->8---
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 0af5250..f3b985d 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -126,6 +126,7 @@ config X86
> select RTC_LIB
> select HAVE_IRQ_EXIT_ON_IRQ_STACK if X86_64
> + select HAVE_IRQ_EXIT_ON_IRQ_STACK if X86_32
> ---8<---
applied both to 3.13.9 and 3.14.0 - issue does not happened any longer

Thanks !

'By rebasing the "sched/core" branch on "master" before the merge and going on with the bisection'

Probably off-topic but I'm really interested what did you do in detail ? I'm asking b/c using git for my own and to bisect a remote tree, but I'm not too familiar in bisecting bugs of this kind. Furthermore probably worth an own section in one of the TODO's ?

- --
Toralf FÃrster
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