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From: "Oliver O'Halloran" <oohall@gmail.com>
To: Alistair Popple <alistair@popple.id.au>
Cc: Andrew Donnellan <ajd@linux.ibm.com>,
	Alexey Kardashevskiy <aik@ozlabs.ru>,
	Reza Arbab <arbab@linux.ibm.com>, Greg Kurz <groug@kaod.org>,
	Christophe Lombard <clombard@linux.ibm.com>,
	Andrew Donnellan <andrew.donnellan@au1.ibm.com>,
	Frederic Barrat <fbarrat@linux.ibm.com>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	Alastair D'Silva <alastair@au1.ibm.com>
Subject: Re: [PATCH 01/11] powerpc/powernv/ioda: Fix ref count for devices with their own PE
Date: Mon, 18 Nov 2019 12:24:24 +1100
Message-ID: <CAOSf1CEzb_Y4NdXGNPYTOxTR5w7OtBUU+VY46CR6Ou5kwEhJqA@mail.gmail.com> (raw)
In-Reply-To: <7553103.1BsQ81z3e0@townsend>

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.

> 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.

Oliver

  reply index

Thread overview: 50+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-09 15:45 [PATCH 00/11] opencapi: enable card reset and link retraining Frederic Barrat
2019-09-09 15:45 ` [PATCH 01/11] powerpc/powernv/ioda: Fix ref count for devices with their own PE Frederic Barrat
2019-09-10  0:20   ` Alastair D'Silva
2019-09-26 16:44   ` Andrew Donnellan
2019-09-26 17:15     ` Frederic Barrat
2019-09-27  6:54       ` Alexey Kardashevskiy
2019-11-12 17:38         ` Frederic Barrat
2019-11-18  1:04           ` Alistair Popple
2019-11-18  1:24             ` Oliver O'Halloran [this message]
2019-11-18  2:36               ` Alistair Popple
2019-11-18 18:21                 ` Frederic Barrat
2019-09-09 15:45 ` [PATCH 02/11] powerpc/powernv/ioda: Protect PE list Frederic Barrat
2019-09-10  0:34   ` Alastair D'Silva
2019-11-19 12:55     ` Frederic Barrat
2019-11-19 13:22       ` Oliver O'Halloran
2019-11-19 14:36         ` Frederic Barrat
2019-11-19  5:55   ` Andrew Donnellan
2019-09-09 15:45 ` [PATCH 03/11] powerpc/powernv/ioda: set up PE on opencapi device when enabling Frederic Barrat
2019-09-10  0:38   ` Alastair D'Silva
2019-09-27 16:43   ` Andrew Donnellan
2019-09-09 15:45 ` [PATCH 04/11] powerpc/powernv/ioda: Release opencapi device Frederic Barrat
2019-09-10  0:56   ` Alastair D'Silva
2019-11-19 17:32     ` Frederic Barrat
2019-11-19  7:08   ` Andrew Donnellan
2019-09-09 15:45 ` [PATCH 05/11] powerpc/powernv/ioda: Find opencapi slot for a device node Frederic Barrat
2019-09-10  0:57   ` Alastair D'Silva
2019-11-19  1:26   ` Andrew Donnellan
2019-11-19 14:33     ` Frederic Barrat
2019-09-09 15:45 ` [PATCH 06/11] pci/hotplug/pnv-php: Remove erroneous warning Frederic Barrat
2019-09-10  0:58   ` Alastair D'Silva
2019-11-19  5:00   ` Andrew Donnellan
2019-09-09 15:45 ` [PATCH 07/11] pci/hotplug/pnv-php: Improve error msg on power state change failure Frederic Barrat
2019-09-10  0:59   ` Alastair D'Silva
2019-11-19  2:23   ` Andrew Donnellan
2019-09-09 15:45 ` [PATCH 08/11] pci/hotplug/pnv-php: Register opencapi slots Frederic Barrat
2019-09-10  1:00   ` Alastair D'Silva
2019-11-19  5:18   ` Andrew Donnellan
2019-11-19 15:15     ` Frederic Barrat
2019-09-09 15:45 ` [PATCH 09/11] pci/hotplug/pnv-php: Relax check when disabling slot Frederic Barrat
2019-09-10  1:00   ` Alastair D'Silva
2019-09-27 15:56   ` Andrew Donnellan
2019-09-09 15:45 ` [PATCH 10/11] pci/hotplug/pnv-php: Wrap warnings in macro Frederic Barrat
2019-09-10  1:03   ` Alastair D'Silva
2019-09-26 17:18   ` Andrew Donnellan
2019-09-09 15:46 ` [PATCH 11/11] ocxl: Add PCI hotplug dependency to Kconfig Frederic Barrat
2019-09-10  1:03   ` Alastair D'Silva
2019-09-26 17:11   ` Andrew Donnellan
2019-09-24  4:24 ` [PATCH 00/11] opencapi: enable card reset and link retraining Alexey Kardashevskiy
2019-09-24  6:39   ` Frederic Barrat
2019-09-25  0:20     ` Alexey Kardashevskiy

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