linux-kernel - Re: Circular lockdep in kvm_reset

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Message-ID: <3496a6a10b2d8693825e733b871938f5@misterjones.org>
Date:   Mon, 13 Mar 2023 14:09:55 +0000
From:   Marc Zyngier <maz@...terjones.org>
To:     Cristian Marussi <cristian.marussi@....com>
Cc:     Oliver Upton <oliver.upton@...ux.dev>,
        Jeremy Linton <jeremy.linton@....com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        David Woodhouse <dwmw@...zon.co.uk>,
        Marc Zyngier <marc.zyngier@....com>
Subject: Re: Circular lockdep in kvm_reset_vcpu() ?

On 2023-03-13 10:09, Cristian Marussi wrote:
> On Sat, Feb 11, 2023 at 12:56:41AM +0000, Oliver Upton wrote:
>> Hi Jeremy,
>> 
> 
> Hi,
> 
>> On Fri, Feb 10, 2023 at 11:46:36AM -0600, Jeremy Linton wrote:
>> > Hi,
>> >
>> > I saw this pop yesterday:
>> 
>> You and me both actually! Shame on me, I spoke off-list about this 
>> with
>> Marc in passing. Thanks for sending along the report.
>> 
>> > [   78.333360] ======================================================
>> > [   78.339541] WARNING: possible circular locking dependency detected
>> > [   78.345721] 6.2.0-rc7+ #19 Not tainted
>> > [   78.349470] ------------------------------------------------------
>> > [   78.355647] qemu-system-aar/859 is trying to acquire lock:
>> > [   78.361130] ffff5aa69269eba0 (&host_kvm->lock){+.+.}-{3:3}, at:
>> > kvm_reset_vcpu+0x34/0x274
>> > [   78.369344]
>> > [   78.369344] but task is already holding lock:
>> > [   78.375182] ffff5aa68768c0b8 (&vcpu->mutex){+.+.}-{3:3}, at:
>> > kvm_vcpu_ioctl+0x8c/0xba0
>> 
>> [...]
>> 
>> > It appears to be triggered by the new commit 42a90008f890a ('KVM: Ensure
>> > lockdep knows about kvm->lock vs. vcpu->mutex ordering rule') which is
>> > detecting the vcpu lock grabbed by kvm_vcpu_ioctl() and then the kvm mutext
>> > grabbed by kvm_reset_vcpu().
>> 
>> Right, this commit gave lockdep what it needed to smack us on the head
>> for getting the locking wrong in the arm64 side.
>> 
>> As gross as it might be, the right direction is likely to have our own
>> lock in kvm_arch that we can acquire while holding the vcpu mutex. 
>> I'll
>> throw a patch at the list once I get done testing it.
>> 
> 
> I just hit this using a v6.3-rc2 and a mainline kvmtool.
> 
> In my case, though, the guest does not even boot if I use more than 1
> vcpu, which
> I suppose triggers effectively the reported possible deadlock, i.e.:
> 
> root/lkvm_master run -c 4 -m 4096 -k /root/Image_guest -d
> /root/disk_debian_buster_guest.img -p "loglevel=8"
>   # lkvm run -k /root/Image_guest -m 4096 -c 4 --name guest-288
> ....<HANGS FOREVER>

Pass earlycon to the guest for a start.

I seriously doubt someone has actually seen a deadlock, because
the issue has been there for at least the past 7 years...

And -rc2 works just fine here.

         M.
-- 
Who you jivin' with that Cosmik Debris?