On Fri, 2013-11-01 at 18:01 -0400, David Miller wrote: > From: Cong Wang <xiyou.wangc...@gmail.com> > Date: Thu, 31 Oct 2013 21:19:16 -0700 > > > 2013年10月30日 下午9:26于 "David Miller" <da...@davemloft.net>写道： > >> > >> We have to provide a softint compatible environment for this callback > >> to run in else everything is completely broken. > >> > >> All these drivers can safely assume softirq safe locking is > >> sufficient, you're suggesting we need to take this hardirq safety and > >> I'm really not willing to allow things to go that far. A lot of > >> effort has been expended precisely to avoid that kind of overhead and > >> cost. > > > > Alright, I am thinking to move netpoll_poll_dev() to a delayed work. > > What if the printk is outputting a message that will help us discover > that work queues are deadlocked? > > You can't delay the message, because every layer of indirection you > add increases the possibility that the message it never seen. You > have to do it synchronously.
As you've said, the ndo_start_xmit and NAPI poll operations are intended to be called in softirq context, so everything that interlocks with them will use spin_lock_bh(). Calling them from hardirq context obviously opens the possibility of a deadlock. How do you expect anyone to solve that? I think that most of the time netpoll doesn't actually call the NAPI poll function, and the driver ndo_start_xmit function doesn't take any locks, so we don't actually hit the deadlock in practice (on mainline kernels - RT is a different story). Obviously, the less machinery netpoll relies on continuing to work, the better, so it should preferably defer to sofirq context rather than workqueue context. I think this means hooking queue_process() into net_tx_action(), and then cutting out much of the rest of netpoll. Ben. -- Ben Hutchings, Staff Engineer, Solarflare Not speaking for my employer; that's the marketing department's job. They asked us to note that Solarflare product names are trademarked. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
