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Subject: Re: [PATCH] s390/vfio-ap: do not use open locks during
 VFIO_GROUP_NOTIFY_SET_KVM notification
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     Halil Pasic <pasic@linux.ibm.com>, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org, pasic@linux.vnet.ibm.com,
        jjherne@linux.ibm.com
References: <20210625220758.80365-1-akrowiak@linux.ibm.com>
 <20210628222923.21a257c8.pasic@linux.ibm.com>
 <25edecce-0795-3b00-a155-bfcc8499f1be@linux.ibm.com>
 <20210701003941.685c524c.pasic@linux.ibm.com>
 <8b742188-d8a2-cf4e-e9de-0ca6f3d829b3@linux.ibm.com>
 <20210705141348.GN4459@nvidia.com>
 <11025fe5-0751-b7ab-9250-b21c10382edd@linux.ibm.com>
 <20210706134939.GX4459@nvidia.com>
From:   Tony Krowiak <akrowiak@linux.ibm.com>
Message-ID: <f958be51-a5fd-a755-f1de-a580f9ccbace@linux.ibm.com>
Date:   Tue, 6 Jul 2021 18:43:39 -0400
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On 7/6/21 9:49 AM, Jason Gunthorpe wrote:
> On Tue, Jul 06, 2021 at 09:39:29AM -0400, Tony Krowiak wrote:
>>
>> On 7/5/21 10:13 AM, Jason Gunthorpe wrote:
>>> On Thu, Jul 01, 2021 at 10:28:52AM -0400, Tony Krowiak wrote:
>>>
>>>>> I think Jason was talking about open coding locks in general.
>>>> That may be so, but his comments were in support of his
>>>> statement that the  mutex + wait_queue did not resolve
>>>> the issue reported vai the lockdep splat because it turned
>>>> off lockdep.
>>> Rgiht, if this used to be proper locks and lockdep complained then
>>> whatever potential deadlock it found is not magically removed by going
>>> to a wait_queue. It just removes the lockdep annotations that would
>>> identify the issue early.
>>>
>>> This is why people should not open code locks, it completely defeats
>>> lockdep. That alone is merit enough for this patch.
>> When you use the phrase "open code locks", to what are you
>> specifically referring? I am confused by the use of the phrase
>> "open code" in this context because open coding, at least as
>> I understand it, has to do with data analysis.
> "open code" here means you write the algorithm of a standard lock in
> your own functions instead of calling the standard library.
>
> Testing/setting the busy and sleeping on a wait_event is exactly a
> standard lock.
>
> Ie if I write
>
>     for (len = 0; str[len] != 0; len++)
>             ;
>
> Then I have open coded strlen()
>
> Jason

Thanks for the explanation.