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During my recent work on the rtnl lock deadlock in the IPoIB driver, I saw that even once I fixed the apparent races for a single device, as soon as that device had any children, new races popped up. It turns out that this is because no matter how well we protect against races on a single device, the fact that all devices use the same workqueue, and flush_workqueue() flushes *everything* from that workqueue, we can have one device in the middle of a down and holding the rtnl lock and another totally unrelated device needing to run mcast_restart_task, which wants the rtnl lock and will loop trying to take it unless is sees its own FLAG_ADMIN_UP flag go away. Because the unrelated interface will never see its own ADMIN_UP flag drop, the interface going down will deadlock trying to flush the queue. There are several possible solutions to this problem: Make carrier_on_task and mcast_restart_task try to take the rtnl for some set period of time and if they fail, then bail. This runs the real risk of dropping work on the floor, which can end up being its own separate kind of deadlock. Set some global flag in the driver that says some device is in the middle of going down, letting all tasks know to bail. Again, this can drop work on the floor. I suppose if our own ADMIN_UP flag doesn't go away, then maybe after a few tries on the rtnl lock we can queue our own task back up as a delayed work and return and avoid dropping work on the floor that way. But I'm not 100% convinced that we won't cause other problems. Or the method this patch attempts to use, which is when we bring an interface up, create a workqueue specifically for that interface, so that when we take it back down, we are flushing only those tasks associated with our interface. In addition, keep the global workqueue, but now limit it to only flush tasks. In this way, the flush tasks can always flush the device specific work queues without having deadlock issues. Signed-off-by: Doug Ledford <dledford@redhat.com> Signed-off-by: Roland Dreier <roland@purestorage.com> |
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