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Subject: Re: [PATCH 01/11] powerpc/powernv/ioda: Fix ref count for devices
 with their own PE
To: Alistair Popple <alistair@popple.id.au>, linuxppc-dev@lists.ozlabs.org
References: <20190909154600.19917-1-fbarrat@linux.ibm.com>
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From: Frederic Barrat <fbarrat@linux.ibm.com>
Date: Mon, 18 Nov 2019 19:21:08 +0100
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Cc: Andrew Donnellan <ajd@linux.ibm.com>, Alexey Kardashevskiy <aik@ozlabs.ru>,
 Greg Kurz <groug@kaod.org>, Christophe Lombard <clombard@linux.ibm.com>,
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Le 18/11/2019 à 03:36, Alistair Popple a écrit :
> On Monday, 18 November 2019 12:24:24 PM AEDT Oliver O'Halloran wrote:
>> On Mon, Nov 18, 2019 at 12:06 PM Alistair Popple <alistair@popple.id.au>
> wrote:
>>>
>>> On Wednesday, 13 November 2019 4:38:21 AM AEDT Frederic Barrat wrote:
>>>>
>>>> However, one question is whether this patch breaks nvlink and if nvlink
>>>> assumes the devices won’t go away because we explicitly take a reference
>>>> forever. In npu_dma.c, there are 2 functions which allow to find the GPU
>>>> associated to a npu device, and vice-versa. Both functions return a
>>>> pointer to a struct pci_dev, but they don’t take a reference on the
>>>> device being returned. So that seems dangerous. I’m probably missing
>>>> something.
>>>>
>>>> Alexey, Alistair: what, if anything, guarantees, that the npu or gpu
>>>> devices stay valid. Is it because we simply don’t provide any means to
>>>> get rid of them ? Otherwise, don’t we need the callers of
>>>> pnv_pci_get_gpu_dev() and pnv_pci_get_npu_dev() to worry about reference
>>>> counting ? I’ve started looking into it and the changes are scary, which
>>>> explains Greg’s related commit 02c5f5394918b.
>>>
>>> To be honest the reference counting looks like it has evolved into
> something
>>> quite suspect and I don't think you're missing anything. In practice
> though we
>>> likely haven't hit any issues because the original callers didn't store
>>> references to the pdev which would make the window quite small (although
> the
>>> pass through work may have changed that). And as you say there simply
> wasn't
>>> any means to get rid of them anyway (EEH, hotplug, etc. has never been
>>> implemented or supported for GPUs and all sorts of terrible things happen
> if
>>> you try).
>>
>> In other words: leaking a ref is the only safe thing to do.
> 
> Correct.
> 
>>> The best solution would likely be to review the reference counting and to
>>> teach callers of get_*_dev() to call pci_put_dev(), etc.
>>
>> The issue is that the two callers of get_pci_dev() are non-GPL
>> exported symbols so we don't know what's calling them or what their
>> expectations are. We be doing whatever makes sense for OpenCAPI and if
>> that happens to cause a problem for someone else, then they can deal
>> with it.
> 
> The issue isn't that it's exported non-GPL vs. GPL, rather that there are
> probably out-of-tree modules like the NVIDIA driver using it. However as you
> say supporting out-of-tree modules is not generally a concern for kernel
> developers so we certainly shouldn't let that prevent us doing a fix.


Thanks Alistair and Oliver. I'll bite the bullet and try do the right 
thing wrt ref counting in npu-dma.c

   Fred

> - Alistair
> 
>> Oliver
> 
> 
> 
>