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

[Jeffrey Hugo <jhugo@codeaurora.org>]

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@codeaurora.org>
> 
> 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@linux-foundation.org>
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.