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

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Date:   Tue, 6 Jul 2021 10:49:39 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Tony Krowiak <akrowiak@...ux.ibm.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 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