If a request comes in to an address range managed by a userspace driver
i.e. <linux/firewire-cdev.h> client, the card instance of request and
response may differ from the card instance of the client device.
Therefore we need to take a reference of the card until the response was
sent.
I thought about putting the reference counting into core-transaction.c,
but the various high-level drivers besides cdev clients (firewire-net,
firewire-sbp2, firedtv) use the card pointer in their fw_address_handler
address_callback method only to look up devices of which they already
hold the necessary references. So this seems to be a specific
firewire-cdev issue which is better addressed locally.
We do not need the reference
- in case of FCP_REQUEST or FCP_RESPONSE requests because then the
firewire-core will send the split transaction response for us
already in the context of the request handler,
- if it is the same card as the client device's because we hold a
card reference indirectly via teh client->device reference.
To keep things simple, we take the reference nevertheless.
Jay Fenlason wrote:
> there's no way for the core to tell cdev "this card is gone,
> kill any inbound transactions on it", while cdev holds the transaction
> open until userspace issues a SEND_RESPONSE ioctl, which may be a very,
> very long time. But when it does, it calls fw_send_response(), which
> will dereference the card...
>
> So how unhappy are we about userspace potentially holding a fw_card
> open forever?
While termination of inbound transcations at card removal could be
implemented, it is IMO not worth the effort. Currently, the effect of
holding a reference of a card that has been removed is to block the
process that called the pci_remove of the card. This is
- either a user process ran by root. Root can find and kill processes
that have /dev/fw* open, if desired.
- a kernel thread (which one?) in case of hot removal of a PCCard or
ExpressCard.
The latter case could be a problem indeed. firewire-core's card
shutdown and card release should probably be improved not to block in
shutdown, just to defer freeing of memory until release.
This is not a new problem though; the same already always happens with
the client->device->card without the need of inbound transactions or
other special conditions involved, other than the client not closing the
file.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
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