Re: [lwip-users] Socket read/write and threads

[Yoav Nissim]

On 11/01/10 16:50 PM, Simon Goldschmidt wrote:
> Yoav Nissim wrote:
>   
>> Simon,
>> Is the half duplex limitation only a socket implementation issue? other
>> than the missing locks, what else is missing? using the netconn API
>> seems to result in the same problem.
>>     
> Well, the problem really lies in the netconn API (and the sockets API is 
> implemented by more or less directly using the netconn API).
>
>   
>> Isn't the TCPIP thread performing operations one at a time anyway?
>>     
> It does (though not always). But the problem here is that both operations use 
> the same 'finished' semaphore to know when the tcpip_thread has finished the 
> operation:
>
> 1. Thread 1 calls read
> 2. Thread 2 calls write
> 3. tcpip_thread executes the read
> 4. tcpip_thread sets finished-semaphore for read
> -> At this point, it's unpredictable which thread is woken (read or write) as 
> both are pending on the same semaphore.
>
> Plus, write operations may be splitted into multiple time slices if the send 
> window or buffer space does not allow sending all data at a time.
>
>   
Yes. That's similar to the analysis I posted a while back after
encountering the problem, only in my case it happened only when the send
was larger than tcp_sndbuf().
You then replied, and rightfully so I believe, that supporting those
scenarios would take the 'lw' out of lwIP.

I wonder though - if these locks were taken care of, reading and writing
to a single socket could be simultaneous but, would lwIP be full duplex
'on the wire' as you've said?


> Simon
>   

-- 
Yoav Nissim
Software Engineer, Software Tools Division
Jungo Software Technologies
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