Messages in this thread | | | From | Jonas Bonn <> | Subject | Packet gets stuck in NOLOCK pfifo_fast qdisc | Date | Wed, 9 Oct 2019 08:46:19 +0200 |
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Hi,
The lockless pfifo_fast qdisc has an issue with packets getting stuck in the queue. What appears to happen is:
i) Thread 1 holds the 'seqlock' on the qdisc and dequeues packets. ii) Thread 1 dequeues the last packet in the queue. iii) Thread 1 iterates through the qdisc->dequeue function again and determines that the queue is empty.
iv) Thread 2 queues up a packet. Since 'seqlock' is busy, it just assumes the packet will be dequeued by whoever is holding the lock.
v) Thread 1 releases 'seqlock'.
After v), nobody will check if there are packets in the queue until a new packet is enqueued. Thereby, the packet enqueued by Thread 2 may be delayed indefinitely.
What, I think, should probably happen is that Thread 1 should check that the queue is empty again after releasing 'seqlock'. I poked at this, but it wasn't obvious to me how to go about this given the way the layering works here. Roughly:
qdisc_run_end() { ... spin_unlock(seqlock); if (!qdisc_is_empty(qdisc)) qdisc_run(); ... }
Calling qdisc_run() from qdisc_run_end() doesn't feel right!
There's a qdisc->empty property (and qdisc_is_empty() relies on it) but it's not particularly useful in this case since there's a race in setting this property which makes it not quite reliable.
Hope someone can shine some light on how to proceed here.
/Jonas
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