Kan Yan <kyan@xxxxxxxxxx> writes: > Thanks for the review. I will pick up your new patches and give it a > try tomorrow. > >> Why is this setting sta and device limits to the same value? > > local->aql_txq_limit_low is not the per device limit, but the default > txq_limit for all STAs. Individual stations can be configured with > non-default value via debugfs entry > "netdev:interface_name_x/stations/mac_addr_x/airtime". "aql_threshold" > is the device limit for switching between the lower and higher per > station queue limit. Oh, right, I see. But in that case, should writing the default really stomp on all the per-station values? If I set the value of a station, I wouldn't expect it to change just because I changed the default value afterwards? >> Also, are you sure we won't risk write tearing when writing 32-bit >> values without locking on some architectures? > > Does mac80211 ever runs in any 16-bit architectures? Even in an > architecture that write to 32-bit value is not atomic, I don't think > there is any side-effect for queue limit get wrong transiently in rare > occasions. Besides, the practical value of those queue limits should > always fit into 16 bits. I'm not sure about the platform characteristics of all the weird tiny MIPS boxes that run OpenWrt; which is why I'm vary of making any assumptions that it is safe :) But yeah, I suppose you're right that since we're just setting the limit, it is not going to be a huge concern here... >> I don't think this is right; another thread could do atomic_inc() >> between the atomic_read() and atomic_set() here, in which case this >> would clobber the other value. >> I think to get this right the logic would need to be something like >> this: >> retry: >> old = atomic_read(&sta->airtime[ac].aql_tx_pending); >> if (warn_once(tx_airtime > old)) >> new = 0; >> else >> new = old - tx_airtime; >> if (atomic_cmpxchg(&sta->airtime[ac].aql_tx_pending, old, new) != old) >> goto retry; >> (or use an equivalent do/while). > > That's indeed not right. However, if a potential aql_tx_pending > underflow case is detected here (It should never happen), reset it to > 0 maybe not the best remedy anyway. I think it is better just > WARN_ONCE() and skip updating aql_tx_pending all together, so the > retry or loop can be avoided here. What do you think? If we don't reset the value to zero may end up with a device that is unable to transmit. Better to reset it I think, even if this is never supposed to happen... -Toke
