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Date:   Thu, 12 Jan 2023 23:29:53 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Matthew Rosato <mjrosato@linux.ibm.com>
Cc:     Alex Williamson <alex.williamson@redhat.com>, pbonzini@redhat.com,
        jgg@nvidia.com, cohuck@redhat.com, farman@linux.ibm.com,
        pmorel@linux.ibm.com, borntraeger@linux.ibm.com,
        frankja@linux.ibm.com, imbrenda@linux.ibm.com, david@redhat.com,
        akrowiak@linux.ibm.com, jjherne@linux.ibm.com, pasic@linux.ibm.com,
        zhenyuw@linux.intel.com, zhi.a.wang@intel.com,
        linux-s390@vger.kernel.org, kvm@vger.kernel.org,
        intel-gvt-dev@lists.freedesktop.org,
        intel-gfx@lists.freedesktop.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] vfio: fix potential deadlock on vfio group lock
Message-ID: <Y8CX8YwT/T9v4U/D@google.com>
References: <20230112203844.41179-1-mjrosato@linux.ibm.com>
 <20230112140517.6db5b346.alex.williamson@redhat.com>
 <bce7912a-f904-b5a3-d234-c3e2c42d9e54@linux.ibm.com>
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In-Reply-To: <bce7912a-f904-b5a3-d234-c3e2c42d9e54@linux.ibm.com>
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On Thu, Jan 12, 2023, Matthew Rosato wrote:
> On 1/12/23 4:05 PM, Alex Williamson wrote:
> > On Thu, 12 Jan 2023 15:38:44 -0500
> > Matthew Rosato <mjrosato@linux.ibm.com> wrote:
> >> @@ -344,6 +345,35 @@ static bool vfio_assert_device_open(struct vfio_device *device)
> >>  	return !WARN_ON_ONCE(!READ_ONCE(device->open_count));
> >>  }
> >>  
> >> +static bool vfio_kvm_get_kvm_safe(struct kvm *kvm)
> >> +{
> >> +	bool (*fn)(struct kvm *kvm);
> >> +	bool ret;
> >> +
> >> +	fn = symbol_get(kvm_get_kvm_safe);
> >> +	if (WARN_ON(!fn))

In a related vein to Alex's comments about error handling, this should not WARN.
WARNing during vfio_kvm_put_kvm() makes sense, but the "get" is somewhat blind.

> >> +		return false;
> >> +
> >> +	ret = fn(kvm);
> >> +
> >> +	symbol_put(kvm_get_kvm_safe);
> >> +
> >> +	return ret;
> >> +}
> >> +
> >> +static void vfio_kvm_put_kvm(struct kvm *kvm)
> >> +{
> >> +	void (*fn)(struct kvm *kvm);
> >> +
> >> +	fn = symbol_get(kvm_put_kvm);
> >> +	if (WARN_ON(!fn))
> >> +		return;
> >> +
> >> +	fn(kvm);
> >> +
> >> +	symbol_put(kvm_put_kvm);
> >> +}