On Tue, 24 Jun 2014 15:07:05 -0500, Nathan Fontenot [off-list ref] wrote:

On 06/23/2014 09:58 AM, Grant Likely wrote:

quoted

On Thu, 19 Jun 2014 11:33:20 +0300, Pantelis Antoniou [off-list ref] wrote:

quoted

Hi Grant,

CCing Thomas Gleixner & Steven Rostedt, since they might have a few
ideas...

On Jun 18, 2014, at 11:07 PM, Grant Likely wrote:

quoted

Hi Nathan and Tyrel,

I'm looking into lifecycle issues on nodes modified by OF_DYNAMIC, and
I'm hoping you can help me. Right now, pseries seems to be the only
user of OF_DYNAMIC, but making OF_DYNAMIC work has a huge impact on
the entire kernel because it requires all DT code to manage reference
counting with iterating over nodes. Most users simply get it wrong.
Pantelis did some investigation and found that the reference counts on
a running kernel are all over the place. I have my doubts that any
code really gets it right.

The problem is that users need to know when it is appropriate to call
of_node_get()/of_node_put(). All list traversals that exit early need
an extra call to of_node_put(), and code that is searching for a node
in the tree and holding a reference to it needs to call of_node_get().

In hindsight it appears that drivers just can't get the lifecycle right.
So we need to simplify things.

quoted

I've got a few pseries questions:
- What are the changes being requested by pseries firmware? Is it only
CPUs and memory nodes, or does it manipulate things all over the tree?
- How frequent are the changes? How many changes would be likely over
the runtime of the system?
- Are you able to verify that removed nodes are actually able to be
freed correctly? Do you have any testcases for node removal?

I'm thinking very seriously about changing the locking semantics of DT
code entirely so that most users never have to worry about
of_node_get/put at all. If the DT code is switched to use rcu
primitives for tree iteration (which also means making DT code use
list_head, something I'm already investigating), then instead of
trying to figure out of_node_get/put rules, callers could use
rcu_read_lock()/rcu_read_unlock() to protect the region that is
searching over nodes, and only call of_node_get() if the node pointer
is needed outside the rcu read-side lock.

I'd really like to be rid of the node reference counting entirely, but
I can't figure out a way of doing that safely, so I'd settle for
making it a lot easier to get correct.

Since we're going about changing things, how about that devtree_lock?

I believe rcu would pretty much eliminate the devtree_lock entirely. All
modifiers would need to grab a mutex to ensure there is only one writer
at any given time, but readers would have free reign to parse the tree
however they like.

DT writers would have to follow some strict rules about how to handle
nodes that are removed (ie. don't modify or of_node_put() them until
after rcu is syncronized), but the number of writers is very small and
we have control of all of them.

quoted

We're using a raw_spinlock and we're always taking the lock with
interrupts disabled.

If we're going to make DT changes frequently during normal runtime
and not only during boot time, those are bad for any kind of real-time
performance.

So the question is, do we really have code that access the live tree
during atomic sections?  Is that something we want? Enforcing this
will make our lives easier, and we'll get the change to replace
that spinlock with a mutex.

Yes, I believe the powerpc CPU hotplug code accesses the DT in atomic
sections. I cannot put my finger on the exact code however. Nathan might
know better. But, if I'm right, the whole problem goes away with RCU.

I went back through the cpu hotplug code. we do update the DT during cpu
hotplug but I don't see it happening during atomic sections.

The code is in arch/powerpc/platforms/pseries/dlpar.c

Great, thanks,

By the way, notifiers currently get sent before any updates are applied
to the tree. I want to change it so that the notifier gets sent
afterwards. Does that work for you? I've looked through all the users
and aside from a stupid block of code in arch/powerpc/kernel/prom.c
which does things that should be done by of_attach_node(), it looks like
everything should be fine.

g.
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