On Mon, Jul 9, 2012 at 12:23 PM, Johannes Berg <johannes@xxxxxxxxxxxxxxxx> wrote: > On Mon, 2012-07-09 at 12:15 +0300, Arik Nemtsov wrote: >> On Mon, Jul 9, 2012 at 12:10 PM, Johannes Berg >> <johannes@xxxxxxxxxxxxxxxx> wrote: >> > On Mon, 2012-07-09 at 11:48 +0300, Arik Nemtsov wrote: >> >> On Mon, Jul 9, 2012 at 10:59 AM, Johannes Berg >> >> <johannes@xxxxxxxxxxxxxxxx> wrote: >> >> > On Sun, 2012-07-08 at 19:27 +0300, Arik Nemtsov wrote: >> >> >> On Sat, Jul 7, 2012 at 12:05 AM, Johannes Berg >> >> >> <johannes@xxxxxxxxxxxxxxxx> wrote: >> >> >> > From: Johannes Berg <johannes.berg@xxxxxxxxx> >> >> >> > >> >> >> > Making the scan_sdata pointer usable with RCU makes >> >> >> > it possible to dereference it in the RX path to see >> >> >> > if a received frame actually matches the interface >> >> >> > that is scanning. This is just preparations, making >> >> >> > the pointer __rcu. >> >> >> >> >> >> I noticed no synchronize_rcu() in the start/stop scan calls. Good/bad idea? >> >> > >> >> > Well, start() certainly wouldn't need it since you'd only get NULL :-) >> >> > >> >> > stop() in theory could use it, but it doesn't actually matter because as >> >> > long as the interface still exists the pointer is valid. We don't free >> >> > the interface in scan stop, so we don't need to make sure that the >> >> > pointer is cleared before we continue. And in the case that we *do* in >> >> > fact clear the interface (when it's going down) we have synchronize_rcu >> >> > already in those code paths due to say the interface list with RCU >> >> > protection. >> >> >> >> I meant protecting these (in patch 2/3): >> >> >> >> - local->sched_scanning, >> >> + rcu_dereference_protected(local->sched_scan_sdata, >> >> + lockdep_is_held(&local->mtx)), >> >> >> >> The check is obviously racy here, but it was racy before as well I guess. >> >> I'm not sure why something line test_bit(SCHED_SCANNING) wasn't used >> >> in these places. >> > >> > I don't think I understand what you're trying to say ... why is this >> > racy? We hold the mutex that we always hold when we assign the pointer. >> >> I mean this check in ieee80211_rx_h_passive_scan(): >> >> if (test_bit(SCAN_HW_SCANNING, &local->scanning) || >> test_bit(SCAN_SW_SCANNING, &local->scanning) || >> test_bit(SCAN_ONCHANNEL_SCANNING, &local->scanning) || >> local->sched_scanning) >> return ieee80211_scan_rx(rx->sdata, skb); >> >> since this is RCU, the pointer might be there a while longer after the >> scan finished.. > > Oh. I was looking at the code after patch 3 and this no longer > exists ;-) > > But then my first argument applies -- as long as the interface is there, > the pointer is OK, and when the interface is removed we need to remove > it from the RCU-managed interface list so need to synchronize_rcu() > already. No? The add/remove interface part is covered, yes. What happens when starting/stopping sched scan? The rcu pointer is removed in ieee80211_request_sched_scan_stop(), but we may still think we are sched scanning for a while inside ieee80211_rx_h_passive_scan(). Probably nothing too bad will happen though.. -- To unsubscribe from this list: send the line "unsubscribe linux-wireless" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
