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SubjectRe: [BUG] Deadlock due due to interactions of block, RCU, and cpu offline
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On 8/22/2017 10:12 AM, Paolo Bonzini wrote:
> On 20/08/2017 22:56, Paul E. McKenney wrote:
>>> 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.
>>
>> My guess would be an undo of the change that provoked the problem
>> in the first place. Did you try bisecting within the above group
>> of commits?
>>
>> Either way, CCing Paolo for his thoughts?
>
> There is "kvm: async_pf: fix rcu_irq_enter() with irqs enabled", but it
> would have caused splats, not deadlocks.
>
> If you are using nested virtualization, "KVM: async_pf: avoid async pf
> injection when in guest mode" can be a wildcard, but only if you have
> memory pressure.
>
> My bet is still on the former changing the timing just a little bit.
>
> Paolo
>

I'm sorry, I must have done the bisect incorrectly.

I attempted to bisect the KVM changes from the merge, but was seeing
that the issue didn't repro with any of them. I double checked the
merge commit, and found it did not introduce a "fix".

I redid the bisect, and it identified the following change this time. I
double checked that reverting the change reintroduces the deadlock, and
cherry-picking the change onto 4.12-rc4 (known to exhibit the issue)
causes the issue to disappear. I'm pretty sure (knock on wood) that the
bisect result is actually correct this time.

commit 6460495709aeb651896bc8e5c134b2e4ca7d34a8
Author: James Wang <jnwang@suse.com>
Date: Thu Jun 8 14:52:51 2017 +0800

Fix loop device flush before configure v3

While installing SLES-12 (based on v4.4), I found that the installer
will stall for 60+ seconds during LVM disk scan. The root cause was
determined to be the removal of a bound device check in loop_flush()
by commit b5dd2f6047ca ("block: loop: improve performance via blk-mq").

Restoring this check, examining ->lo_state as set by loop_set_fd()
eliminates the bad behavior.

Test method:
modprobe loop max_loop=64
dd if=/dev/zero of=disk bs=512 count=200K
for((i=0;i<4;i++))do losetup -f disk; done
mkfs.ext4 -F /dev/loop0
for((i=0;i<4;i++))do mkdir t$i; mount /dev/loop$i t$i;done
for f in `ls /dev/loop[0-9]*|sort`; do \
echo $f; dd if=$f of=/dev/null bs=512 count=1; \
done

Test output: stock patched
/dev/loop0 18.1217e-05 8.3842e-05
/dev/loop1 6.1114e-05 0.000147979
/dev/loop10 0.414701 0.000116564
/dev/loop11 0.7474 6.7942e-05
/dev/loop12 0.747986 8.9082e-05
/dev/loop13 0.746532 7.4799e-05
/dev/loop14 0.480041 9.3926e-05
/dev/loop15 1.26453 7.2522e-05

Note that from loop10 onward, the device is not mounted, yet the
stock kernel consumes several orders of magnitude more wall time
than it does for a mounted device.
(Thanks for Mike Galbraith <efault@gmx.de>, give a changelog review.)

Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: James Wang <jnwang@suse.com>
Fixes: b5dd2f6047ca ("block: loop: improve performance via blk-mq")
Signed-off-by: Jens Axboe <axboe@fb.com>

Considering the original analysis of the issue, it seems plausible that
this change could be fixing it.

--
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.

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