Re: [PATCH v8 2/4] i2c-smbus: add SMBus Host Notify support

On Mon, 18 Jul 2016 18:35:19 +0200, Benjamin Tissoires wrote:

On Jul 18 2016 or thereabouts, Jean Delvare wrote:

quoted

But what happens on i2c_adapter removal? What prevents the following
sequence from happening?

1* A Host Notify event happens.
2* The event is handled and queued by i2c_handle_smbus_host_notify().
3* Someone tears down the underlying i2c_adapter (for example "rmmod
   i2c-i801".)
4* The workqueue is processed, accessing memory which has already been
   freed.

Of course it would be back luck, but that's pretty much the definition
of a race condition ;-)

Yes, you are right :(
Sorry for not doing things properly :/

No worry. Bugs happen everywhere, we find them and fix them. That's
part of the process. If we only submit patches which we are 100%
certain are perfect, we never submit anything. I know something about
that...

quoted

To be on the safe side, don't we need a teardown function in i2c-smbus,
that could be called before i2c_del_adapter, which would remove the
host notify handle and flush the workqueue?

I was thinking at adding a devm action on the release of the struct
smbus_host_notify, but it's actually a bad idea because some other
resources (children moslty) might already be released when the devres
action will be called.

I think it might be easier to add a i2c_remove_host_notify() (or such)
which would make sure we call the cancel_work_sync() function. It would
be the responsibility of the caller to call it once
i2c_setup_smbus_host_notify() has been called. I'd say it has the
advantage of not adding any hidden data in the adapter to the cost of a
small pain in the adapter driver.

That's what I had in mind as well, but I'm open to any option which
solves the problem really.

-- 
Jean Delvare
SUSE L3 Support