Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH] s390/vfio-ap: do not open code locks for
 VFIO_GROUP_NOTIFY_SET_KVM notification
To:     Halil Pasic <pasic@linux.ibm.com>
Cc:     linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        borntraeger@de.ibm.com, cohuck@redhat.com,
        pasic@linux.vnet.ibm.com, jjherne@linux.ibm.com, jgg@nvidia.com,
        alex.williamson@redhat.com, kwankhede@nvidia.com,
        frankja@linux.ibm.com, david@redhat.com, imbrenda@linux.ibm.com,
        hca@linux.ibm.com
References: <20210707154156.297139-1-akrowiak@linux.ibm.com>
 <20210713013815.57e8a8cb.pasic@linux.ibm.com>
From:   Tony Krowiak <akrowiak@linux.ibm.com>
Message-ID: <5dd3cc05-f789-21a3-50c7-ee80d850a105@linux.ibm.com>
Date:   Tue, 13 Jul 2021 09:48:01 -0400
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.8.1
MIME-Version: 1.0
In-Reply-To: <20210713013815.57e8a8cb.pasic@linux.ibm.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
Content-Language: en-US
Precedence: bulk


On 7/12/21 7:38 PM, Halil Pasic wrote:
> On Wed,  7 Jul 2021 11:41:56 -0400
> Tony Krowiak <akrowiak@linux.ibm.com> wrote:
>
>> It was pointed out during an unrelated patch review that locks should not
>> be open coded - i.e., writing the algorithm of a standard lock in a
>> function instead of using a lock from the standard library. The setting and
>> testing of the kvm_busy flag and sleeping on a wait_event is the same thing
>> a lock does. Whatever potential deadlock was found and reported via the
>> lockdep splat was not magically removed by going to a wait_queue; it just
>> removed the lockdep annotations that would identify the issue early
> Did you change your opinion since we last talked about it? This reads to
> me like we are deadlocky without this patch, because of the last
> sentence.

The words are a direct paraphrase of Jason G's responses to my
query regarding what he meant by open coding locks. I
am choosing to take his word on the subject and remove the
open coded locks.

Having said that, we do not have a deadlock problem without
this patch. If you recall, the lockdep splat occurred ONLY when
running a Secure Execution guest in a CI environment. Since
AP is not yet supported for SE guests, there is no danger of
a lockdep splat occurring in a customer environment. Given
Jason's objections to the original solution (i.e., kvm_busy flag
and wait queue), I decided to replace the so-called open
coded locks.

>
> Regards,
> Halil