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From: "Oliver O'Halloran" <oohall@gmail.com>
Date: Mon, 18 Nov 2019 12:24:24 +1100
Message-ID: <CAOSf1CEzb_Y4NdXGNPYTOxTR5w7OtBUU+VY46CR6Ou5kwEhJqA@mail.gmail.com>
Subject: Re: [PATCH 01/11] powerpc/powernv/ioda: Fix ref count for devices
 with their own PE
To: Alistair Popple <alistair@popple.id.au>
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On Mon, Nov 18, 2019 at 12:06 PM Alistair Popple <alistair@popple.id.au> wr=
ote:
>
> 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=E2=80=99t go away because we explicitly take a =
reference
> > forever. In npu_dma.c, there are 2 functions which allow to find the GP=
U
> > associated to a npu device, and vice-versa. Both functions return a
> > pointer to a struct pci_dev, but they don=E2=80=99t take a reference on=
 the
> > device being returned. So that seems dangerous. I=E2=80=99m probably mi=
ssing
> > something.
> >
> > Alexey, Alistair: what, if anything, guarantees, that the npu or gpu
> > devices stay valid. Is it because we simply don=E2=80=99t provide any m=
eans to
> > get rid of them ? Otherwise, don=E2=80=99t we need the callers of
> > pnv_pci_get_gpu_dev() and pnv_pci_get_npu_dev() to worry about referenc=
e
> > counting ? I=E2=80=99ve started looking into it and the changes are sca=
ry, which
> > explains Greg=E2=80=99s related commit 02c5f5394918b.
>
> To be honest the reference counting looks like it has evolved into someth=
ing
> quite suspect and I don't think you're missing anything. In practice thou=
gh 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 was=
n'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