Re: [PATCH] Export current_is_keventd() for libphy

Maciej W. Rozycki
2006-12-06T12:31:21Z

On Tue, 5 Dec 2006, Andy Fleming wrote:

> We need to make sure there are no more pending phy_change() invocations in the
> work queue, this is true, however I'm not convinced that this avoids the
> problem. And now that I come back to this email after Linus's response, let
> me add that I agree with his suggestion. I still don't think it solves the
> original problem, though. Unless I'm missing something, Maciej's suggested
> fix (having the driver invoke phy_stop_interrupts() from a work queue) doesn't
> stop the race:
>
> * Schedule stop_interrupts and freeing of memory.
> * interrupt occurs, and schedules phy_change
> * work_queue triggers, and stop_interrupts is invoked. It doesn't call
> flush_scheduled_work, because it's being called from keventd.
> * The invoker of stop_interrupts (presumably some function in the driver)
> frees up memory, including the phy_device.
> * phy_change is invoked() from the work queue, and starts accessing freed
> memory

This is not going to happen with my other changes to the file applied.
The reason is at the time phy_stop_interrupts() is called phy_stop() has
already run and switched the state of the PHY being handled to PHY_HALTED.
As a result any subsequent calls to phy_interrupt() that might have
happened after phy_stop() have not scheduled calls to phy_change() for
this PHY as will not any that may happen up until free_irq() have
unregistered the interrupt for the PHY.

> I suggested this, mostly so that drivers wouldn't have to be aware of this.
> But I'm not quite sure what happens when you unload a module. Does some stuff
> stay behind if needed? If you unload the ethernet driver, that will usually
> remove the bus controller for the PHY, which would prevent any scheduled code
> from accessing the PHY.

Hmm, I am unsure if there is anything that would ensure flushing of the
queue after its stop() call has finished and before a driver is removed
(its module_exit() call is invoked), probably nothing, and that is why I
put explicit flush_scheduled_work() in sb1250-mac.c:sbmac_remove() and the
driver's open() call checks whether a possible previous instance of the
structure used by phy_change() have not been freed yet. There may be a
cleaner way of doing it, but I will have to think about it.

Maciej