linux-kernel - Re: Circular lockdep in kvm_reset

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Message-ID: <ZA72c+TT9epTcvX4@e120937-lin>
Date:   Mon, 13 Mar 2023 10:09:55 +0000
From:   Cristian Marussi <cristian.marussi@....com>
To:     Oliver Upton <oliver.upton@...ux.dev>
Cc:     Jeremy Linton <jeremy.linton@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>, kvmarm@...ts.linux.dev,
        kvmarm@...ts.cs.columbia.edu,
        "linux-kernel@...r.kernel.org" <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 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>

Thanks,
Cristian