tree b7135fa6db9b39b3f351a6030df0131fb0514163
parent d4018d7fa63d25f3e1ecf6949fca6b81a182231a
author Stefan Richter <stefanr@s5r6.in-berlin.de> 1153685760 +0200
committer Stefan Richter <stefanr@s5r6.in-berlin.de> 1158514162 +0200

ieee1394: sbp2: safer initialization of status fifo

Sbp2's copy of the status fifo was cleared when management ORBs or new
command ORBs were prepared.  The latter had potential for a race
condition if the block layer's soft IRQ and the 1394 LLD's interrupt
handler ran on different CPUs.  It would also yield wrong status if a
command was completed with non-zero completion status before other
commands that had zero completion status, and no new command was
enqueued in the meantime.

Now, the status buffer is cleared right before it is written.  Thus it
ends up in the following simpler and safer access pattern:
 - sbp2_alloc_device: allocates and implicitly clears once,
 - sbp2_handle_status_write: clears, writes, and reads,
 - sbp2_query_logins, sbp2_login_device, sbp2_reconnect_device: read.
The latter three do not race with sbp2_handle_status_write because of
how the protocol works.

As a tiny optimization, the first two quadlets of the status never need
to be cleared.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>