26 Jun 2016

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Google SoC lwIP Report Week 5

Going into week 5, I started with a code-complete but very much incorrect implementation of the TDI_SEND and TDI_RECEIVE IRQ handlers. My TCP_CONTEXT data structure and the existing ADDRESS_FILE data structure both did not contain a way to keep track of pending IRQs, so I had no way of keeping track of outstanding pending IRQs and what connection contexts they were supposed to be associated with. Without a clear scheme for keeping track of the information, IRP pointers invariably got lost. As a result, when trying to handle requests resulting from valid Winsock API calls, my driver leaked memory and left hanging pending IRQs. 

To really address this issue, which resulted from my lack of familiarity with Windows driver programming and the TCP protocol in general, I had to come up with a scheme of how to keep track of all the IRQs my driver receives along with which lwIP protocol control block (PCB) they are associated with. 

My first attempt at solving the problem was creating another struct, the TCP_REQUEST, to link IRPs with their associated lwIP PCBs and general purpose address files. Each address file would contain a list of contexts as before, but each context would keep a list of requests it's pending on. As I tried to properly implement this scheme of keeping track of information, I ran into more and more problems with how to tell what state each address file and connection context was in. Every time I fixed one problem, another one seemed to pop up. If I wasn't freeing memory at an improper time, I was failing the correct pointers or overwriting pointers to outstanding requests. As the week dragged on and I continue to be stuck on the same problem of how to keep track of IRQs and their associated addresses, I finally decided to do two things. First, I would add state variables to the address files, the contexts, or both structs. Second, I would draw an actual state diagram before proceeding further. 

At the week's close, I am still in the process of drawing the state diagram. I hope this is the last large structural change I will need to make to my code to make it functionally correct, if still not feature-complete or entirely robust. 

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