On Wed, 2014-02-19 at 16:09 +0100, Stanislaw Gruszka wrote: > On Wed, Feb 19, 2014 at 03:51:43PM +0100, Johannes Berg wrote: > > On Wed, 2014-02-19 at 14:35 +0100, Stanislaw Gruszka wrote: > > > > > > It ultimately makes no difference at all, it just makes this code more > > > > difficult to read and understand. > > > > > > It make difference when queue length value is modified on different CPU > > > and read on different CPU. Without lock you can 'see' old length value > > > on CPU that run ieee80211_tx_h_multicast_ps_buf() for undefined > > > period of time (ok maybe not undefined on x86), and current oldest > > > frame can be not necessarily dropped. > > > > I don't see how that can be true, since the modifications of the queue > > length are under spinlock with the implied memory barriers. > > On processor that read value there is no memory barrier. If you do: > > CPU1 CPU2 > > b = a; > spin_lock(a_lock) > a = 1; ... > spin_unlock(a_lock) > > b = a; > > There is nothing that guarantee that on CPU2 b will be 1. Well, OK, so there's some small window of time where the drop code isn't perfect. Does it matter? I'd argue it doesn't - if you're getting close to the drop case then the client is probably misbehaving anyway. johannes -- 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
