Am 01.06.2022 um 19:13 schrieb Johannes Berg:
On Wed, 2022-06-01 at 15:37 +0000, Benjamin Beichler wrote:Hmm. How did you run into this? Why would a device send many messages and not wait for ACK, but the kernel side actually waits for ACK? What would the use case for that be? Seems a bit odd, if both wait for ACK there shouldn't be an issue? Anyway, I guess I don't mind fixing this regardless of whether I see a use case where it could happen :-)
Here is my (admittedly maybe odd) case: I want to use hwsim over virtio with UML but without time travel (as a precursor for a later version with TT) I modified wmediumd to strip out the scheduler dependency and wrote a very simple simulation, which simply forwards all frames to all radios. Furthermore, I use the usfstl "loop" as main driver to poll all fds without time travel. This leads to the situation, that when a msg is put on the RX-ring of an uml instance, which also sent concurrently a kick (e.g., also trying to send a frame), this creates a deadlock. In the original wmediumd this was handled by kind of a hack, calling the loop implementation to answer the kick, before sending out a call msg. I need to rip out this workaround, because without the usfstl scheduler, it created a deep recursion of the loop implementation with additional problems. Nonetheless, even if this would be kind of an optimization: it is feasible to wait for the ACK asynchronously, as long as it arrives in the same point of simulation time (or as you called it calender). For many uml-instances, which could easily run in parallel, this allows an easier implementation (at least in my planning :-) ). Of course, it would be hard to distinguish, which call-request was acked, but at the end wmediumd (and I also plan to do so) simply aborts when the ack is negative, so the actual corresponding call is not that important to know.
This code changed a bit, you should rebase onto the uml tree's for-next branch.
My bad, I was not expecting someone to change something it that corner of the kernel, I only used the latest master and not the next. I will redo the patch with ease.
+ while (1) { + if (vhost_user_recv_req(vu_dev, &msg.msg, + sizeof(msg.msg.payload) + + sizeof(msg.extra_payload)))prefer to keep the + on the previous line.
It slightly hits the 80 column restriction, but I would also prefer not to break. :-D
That said, my attempt at rebasing this made it all fail completely, maybe you have better luck :) johannes
kind regards Benjamin -- M.Sc. Benjamin Beichler Universität Rostock, Fakultät für Informatik und Elektrotechnik Institut für Angewandte Mikroelektronik und Datentechnik University of Rostock, Department of CS and EE Institute of Applied Microelectronics and CE Richard-Wagner-Straße 31 18119 Rostock Deutschland/Germany phone: +49 (0) 381 498 - 7278 email:benjamin.beich...@uni-rostock.de www:http://www.imd.uni-rostock.de/
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