On Sun, Oct 05, 2014 at 03:13:12PM +0200, Stanislaw Gruszka wrote: > On Sat, Oct 04, 2014 at 07:38:25PM -0400, Mark Asselstine wrote: > > >> The only possibility to skb_under_panic I can see, is that we retransmit > > >> frame and try to align it many times, but alignment should not be needed > > >> once we aligned frame. Hence I'm not sure how those skb_under_panics can > > >> happen. > > > > I am still digging through trying to find a cause for this, without a > > reproducer I am starting to lose hope on finding the cause. > > > > I dug up this old thread > > http://rt2x00.serialmonkey.com/pipermail/users_rt2x00.serialmonkey.com/2010-November/002457.html > > > > I am thinking that the cause is an interaction with 80211mac and the > > rt2x00queue not doing enough to ensure we are sending the skb back in > > the same state as we get it. Looking at rt2x00lib_txdone() we don't > > return headroom that may have been taken for frame alignment and we > > don't account for the extra 4-bytes taken for header_align when the > > payload_align is larger then the header_align while setting up the > > l2pad. > > I think we reserve sufficient space, 4 bytes for payload align plus > 4 bytes for header align. > > > Do you know why rt2x00 doesn't return frame align space back to > > the headroom? > > For performance reasons I guess. > > If we return (payload) aligned skb to mac80211 and mac80211 will > retransmit that skb we will not align it again, since it's already > aligned. But perhaps we really need to return original tx skb to > mac80211, but if so, I would like to know detailed reason why. There is some old patch from Helmut, which remove payload alignment and movement: http://rt2x00.serialmonkey.com/pipermail/users_rt2x00.serialmonkey.com/2011-September/004179.html It seems to be provided by default on OpenWRT and AFICT nobody reported skb underflow panic there. Hence possibly it fix the problem, but still I would like to know why. However if we apply it, I would relax extra TX headroom requirement to 4 bytes, as that maximum what we need with the patch. Stanislaw -- 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
