linux-kernel - Re: [PATCH] s390/vfio-ap: do not use open locks during VFIO_GROUP_NOTIFY_SET

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Message-ID: <f958be51-a5fd-a755-f1de-a580f9ccbace@linux.ibm.com>
Date:   Tue, 6 Jul 2021 18:43:39 -0400
From:   Tony Krowiak <akrowiak@...ux.ibm.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Halil Pasic <pasic@...ux.ibm.com>, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, pasic@...ux.vnet.ibm.com,
        jjherne@...ux.ibm.com
Subject: Re: [PATCH] s390/vfio-ap: do not use open locks during
 VFIO_GROUP_NOTIFY_SET_KVM notification



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.