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authorJohannes Berg <johannes@sipsolutions.net>2009-07-24 11:13:14 -0700
committerJohn W. Linville <linville@tuxdriver.com>2009-07-27 15:19:34 -0400
commit3995bd9332a51b626237d6671cfeb7235e6c1305 (patch)
tree6e707b7586c30684be54c24720c4c7f590e1691f /Documentation/netlabel/draft-ietf-cipso-ipsecurity-01.txt
parent8bae1b2b13beb4cf4c0f119f97640503c2b74b0f (diff)
iwlwifi: fix TX queue race
I had a problem on 4965 hardware (well, probably other hardware too, but others don't survive my stress testing right now, unfortunately) where the driver was sending invalid commands to the device, but no such thing could be seen from the driver's point of view. I could reproduce this fairly easily by sending multiple TCP streams with iperf on different TIDs, though sometimes a single iperf stream was sufficient. It even happened with a single core, but I have forced preemption turned on. The culprit was a queue overrun, where we advanced the queue's write pointer over the read pointer. After careful analysis I've come to the conclusion that the cause is a race condition between iwlwifi and mac80211. mac80211, of course, checks whether the queue is stopped, before transmitting a frame. This effectively looks like this: lock(queues) if (stopped(queue)) { unlock(queues) return busy; } unlock(queues) ... <-- this place will be important there is some more code here drv_tx(frame) The driver, on the other hand, can stop and start queues, which does lock(queues) mark_running/stopped(queue) unlock(queues) [if marked running: wake up tasklet to send pending frames] Now, however, once the driver starts the queue, mac80211 can see that and end up at the marked place above, at which point for some reason the driver seems to stop the queue again (I don't understand that) and then we end up transmitting while the queue is actually full. Now, this shouldn't actually matter much, but for some reason I've seen it happen multiple times in a row and the queue actually overflows, at which point the queue bites itself in the tail and things go completely wrong. This patch fixes this by just dropping the packet should this have happened, and making the lock in iwlwifi cover everything so iwlwifi can't race against itself (dropping the lock there might make it more likely, but it did seem to happen without that too). Since we can't hold the lock across drv_tx() above, I see no way to fix this in mac80211, but I also don't understand why I haven't seen this before -- maybe I just never stress tested it this badly. With this patch, the device has survived many minutes of simultanously sending two iperf streams on different TIDs with combined throughput of about 60 Mbps. Signed-off-by: Johannes Berg <johannes@sipsolutions.net> Signed-off-by: Reinette Chatre <reinette.chatre@intel.com> Signed-off-by: John W. Linville <linville@tuxdriver.com>
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