linux-kernel - Re: [PATCH] of: introduce event tracepoints for dynamic device

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Date:   Wed, 19 Apr 2017 12:30:50 +1000
From:   "Oliver O'Halloran" <oohall@...il.com>
To:     Rob Herring <robh+dt@...nel.org>
Cc:     Tyrel Datwyler <tyreld@...ux.vnet.ibm.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Frank Rowand <frowand.list@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        rostedt@...dmis.org, Ingo Molnar <mingo@...hat.com>,
        Nathan Fontenot <nfont@...ux.vnet.ibm.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH] of: introduce event tracepoints for dynamic device_node lifecyle

On Wed, Apr 19, 2017 at 2:46 AM, Rob Herring <robh+dt@...nel.org> wrote:
> On Mon, Apr 17, 2017 at 7:32 PM, Tyrel Datwyler
> <tyreld@...ux.vnet.ibm.com> wrote:
>> This patch introduces event tracepoints for tracking a device_nodes
>> reference cycle as well as reconfig notifications generated in response
>> to node/property manipulations.
>>
>> With the recent upstreaming of the refcount API several device_node
>> underflows and leaks have come to my attention in the pseries (DLPAR) dynamic
>> logical partitioning code (ie. POWER speak for hotplugging virtual and physcial
>> resources at runtime such as cpus or IOAs). These tracepoints provide a
>> easy and quick mechanism for validating the reference counting of
>> device_nodes during their lifetime.
>
> Not really relevant for this patch, but since you are looking at
> pseries and refcounting, the refcounting largely exists for pseries.
> It's also hard to get right as this type of fix is fairly common. It's
> now used for overlays, but we really probably only need to refcount
> the overlays or changesets as a whole, not at a node level. If you
> have any thoughts on how a different model of refcounting could work
> for pseries, I'd like to discuss it.

One idea I've been kicking around is differentiating short and long
term references to a node. I figure most leaks are due to a missing
of_node_put() within a stack frame so it might be possible to use the
ftrace infrastructure to detect and emit warnings if a short term
reference is leaked. Long term references are slightly harder to deal
with, but they're less common so we can add more detailed reference
tracking there (devm_of_get_node?).

Oliver