Re: [BUG] Deadlock due due to interactions of block, RCU, and cpu offline

[Jeffrey Hugo]

On 6/29/2017 6:18 PM, Paul E. McKenney wrote:
> On Thu, Jun 29, 2017 at 10:29:12AM -0600, Jeffrey Hugo wrote:
> > On 6/27/2017 6:11 PM, Paul E. McKenney wrote:
> > > On Tue, Jun 27, 2017 at 04:32:09PM -0600, Jeffrey Hugo wrote:
> > > > On 6/22/2017 9:34 PM, Paul E. McKenney wrote:
> > > > > On Wed, Jun 21, 2017 at 09:18:53AM -0700, Paul E. McKenney wrote:
> > > > > > No worries, and I am very much looking forward to seeing the results of
> > > > > > your testing.
> > > > >
> > > > > And please see below for an updated patch based on LKML review and
> > > > > more intensive testing.
> > > >
> > > > I spent some time on this today.  It didn't go as I expected.  I
> > > > validated the issue is reproducible as before on 4.11 and 4.12 rcs 1
> > > > through 4.  However, the version of stress-ng that I was using ran
> > > > into constant errors starting with rc5, making it nearly impossible
> > > > to make progress toward reproduction.  Upgrading stress-ng to tip
> > > > fixes the issue, however, I've still been unable to repro the issue.
> > > >
> > > > Its my unfounded suspicion that something went in between rc4 and
> > > > rc5 which changed the timing, and didn't actually fix the issue.  I
> > > > will run the test overnight for 5 hours to try to repro.
> > > >
> > > > The patch you sent appears to be based on linux-next, and appears to
> > > > have a number of dependencies which prevent it from cleanly applying
> > > > on anything current that I'm able to repro on at this time.  Do you
> > > > want to provide a rebased version of the patch which applies to say
> > > > 4.11?  I could easily test that and report back.
> > >
> > > Here is a very lightly tested backport to v4.11.
> >
> > Works for me. Always reproduced the lockup within 2 minutes on stock
> > 4.11.  With the change applied, I was able to test for 2 hours in
> > the same conditions, and 4 hours with the full system and not
> > encounter an issue.
> >
> > Feel free to add:
> > Tested-by: Jeffrey Hugo <jhugo@xxxxxxxxxxxxxx>
>
> Applied, thank you!
>
> > I'm going to go back to 4.12-rc5 and see if I can get either repro
> > the issue, or identify what changed.  Hopefully I can get to
> > linux-next and double check the original version of the change as
> > well.
>
> Looking forward to hearing what you find!
>
> 							Thanx, Paul

According to git bisect, the following is what "changed"

commit 9d0eb4624601ac978b9e89be4aeadbd51ab2c830
Merge: 5faab9e 9bc1f09
Author: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Date:   Sun Jun 11 11:07:25 2017 -0700

    Merge tag 'for-linus' of git://git.kernel.org/pub/scm/virt/kvm/kvm

    Pull KVM fixes from Paolo Bonzini:
     "Bug fixes (ARM, s390, x86)"

    * tag 'for-linus' of git://git.kernel.org/pub/scm/virt/kvm/kvm:
      KVM: async_pf: avoid async pf injection when in guest mode
      KVM: cpuid: Fix read/write out-of-bounds vulnerability in cpuid 
emulation
      arm: KVM: Allow unaligned accesses at HYP
      arm64: KVM: Allow unaligned accesses at EL2
      arm64: KVM: Preserve RES1 bits in SCTLR_EL2
      KVM: arm/arm64: Handle possible NULL stage2 pud when ageing pages
      KVM: nVMX: Fix exception injection
      kvm: async_pf: fix rcu_irq_enter() with irqs enabled
      KVM: arm/arm64: vgic-v3: Fix nr_pre_bits bitfield extraction
      KVM: s390: fix ais handling vs cpu model
      KVM: arm/arm64: Fix isues with GICv2 on GICv3 migration

Nothing really stands out to me which would "fix" the issue.

--
Jeffrey Hugo
Qualcomm Datacenter Technologies as an affiliate of Qualcomm 
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.