On Thu 2016-12-08 21:32:12, Lino Sanfilippo wrote: > Hi, > > On 08.12.2016 00:15, Francois Romieu wrote: > > Lino Sanfilippo <linosanfili...@gmx.de> : > >> The driver uses a private lock for synchronization between the xmit > >> function and the xmit completion handler, but since the NETIF_F_LLTX flag > >> is not set, the xmit function is also called with the xmit_lock held. > >> > >> On the other hand the xmit completion handler first takes the private lock > >> and (in case that the tx queue has been stopped) the xmit_lock, leading > >> to a reverse locking order and the potential danger of a deadlock. > > > > netif_tx_stop_queue is used by: > > 1. xmit function before releasing lock and returning. > > 2. sxgbe_restart_tx_queue() > > <- sxgbe_tx_interrupt > > <- sxgbe_reset_all_tx_queues() > > <- sxgbe_tx_timeout() > > > > Given xmit won't be called again until tx queue is enabled, it's not clear > > how a deadlock could happen due to #1. > > > > > After spending more thoughts on this I tend to agree with you. Yes, we have > the > different locking order for the xmit_lock and the private lock in two > concurrent > threads. And one of the first things one learns about locking is that this is > a > good way to create a deadlock sooner or later. But in our case the deadlock > can only occur if the xmit function and the tx completion handler perceive > different > states for the tx queue, or to be more specific: > the completion handler sees the tx queue in state "stopped" while the xmit > handler > sees it in state "running" at the same time. Only then both functions would > try to > take both locks, which could lead to a deadlock. > > OTOH Pavel said that he actually could produce a deadlock. Now I wonder if > this is caused > by that locking scheme (in a way I have not figured out yet) or if it is a > different issue.
Pavel has some problems, but that's on different hardware.. and it is
possible that it is deadlock (or something else) somewhere else.
Best regards,
Pavel
--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures)
http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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