netdev - Re: [PATCH v1 01/12] IB/core: pass client data to remove() callbacks

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Date:	Wed, 8 Jul 2015 15:34:10 -0600
From:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:	Haggai Eran <haggaie@...lanox.com>
Cc:	Doug Ledford <dledford@...hat.com>, linux-rdma@...r.kernel.org,
	netdev@...r.kernel.org, Liran Liss <liranl@...lanox.com>,
	Guy Shapiro <guysh@...lanox.com>,
	Shachar Raindel <raindel@...lanox.com>,
	Yotam Kenneth <yotamke@...lanox.com>
Subject: Re: [PATCH v1 01/12] IB/core: pass client data to remove() callbacks

On Wed, Jul 08, 2015 at 02:29:10PM -0600, Jason Gunthorpe wrote:
> On Mon, Jun 22, 2015 at 03:42:30PM +0300, Haggai Eran wrote:
> > An ib_client callback that is called with the lists_rwsem locked only for
> > read is protected from changes to the IB client lists, but not from
> > ib_unregister_device() freeing its client data. This is because
> > ib_unregister_device() will remove the device from the device list with
> > lists_rwsem locked for write, but perform the rest of the cleanup,
> > including the call to remove() without that lock.
> 
> I was going to look at this, but, uh.. it seems mangled, doesn't
> apply, doesn't seem fixable from here.

Okay, I see, it sits on top of the patch from Matan's last
posting.. My bad.

Hum... I have to say I don't really like this, changing the ordering
of client_data = NULL with respect to client->remove doesn't seem like
a great idea - and the rds changes look scary to me, at least I
couldn't confidently say they were OK..

And that isn't really the issue - this has nothing to do with
client_data, it is all about not having a callback running when doing
remove.

It looks like the way out of this is to have ib_get_net_dev_by_params
iterate over the client_data_list and use a dedicated flag in that
struct to indicate that client&device combination is
remove-in-progress.

This would be a bit more efficient as well, and I would suggest
passing the context in as an arg to the callback.

client_data_list would change a bit to become write locked first by
write(lists_rwsem), and then second by the spin lock, so holding
read(lists_rwsem) while iterating is enough locking, and you'd hold
lists_rwsem while kfreeing.

Jason
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