Re: [PATCH 2/2] firewire: core: fix race at address_handler unregistration

[Stefan Richter <stefanr@s5r6.in-berlin.de>]

On Feb 18 Stefan Richter wrote:
> Fix the following unlikely but possible race:
> 
> CPU 1                             CPU 2
> ------------------------------------------------------------------------
> AR-request tasklet
>     lookup handler
>                                   unregister handler
> 				  free handler->callback_data or handler
>     call handler->callback
> 
> The application which registered the handler has no way to stop nodes
> sending new requests to their address range, hence cannot prevent this
> race.
> 
> Fix it simply by extending the address_handler_lock-protected region
> from only around the lookup to around both lookup and call.  We only
> need to do so in the exclusive region handler; the FCP region handler
> already holds the lock around the handler->callback call.
> 
> Alas this removes the current ability to execute the callback in
> parallel on different CPUs if it was called for different FireWire cards
> at the same time.  (For a single card, the handler is already
> serialized.)  If this loss of a rather obscure feature is not tolerable,
> a more complex fix would be required:  Add a handler reference counter;
> wait in fw_core_remove_address_handler() for this conter to become zero.

Oh, and the other downside is that the region in which local IRQs are
disabled is extended.  So I guess I should at least the core, maybe also
the application layer drivers, to spin_lock_bh instead, sooner than later.
-- 
Stefan Richter
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