[lwip-users] Re: [lwip] Pbuf Pool

[Kieran Mansley]

On Tue, 9 Apr 2002, James Roth wrote:
> Mumtaz Ahmad wrote:
>
> >Hi
> >
> >I am using lwip on trimedia board . I am facing a problem when the ethernet
> >interface transfers a buffer to lwip .I am experiencing a data loss as
> >POOL_BUFFS are not available all the time . I have tried to change the
> >number of POOL Buffers but with no success . Most failures are due to free
> >lock problem. I tried to understand it but somehow i ahave been unable to
> >understand all this caching mechanism and i donot know why i get pbuff pool
> >locked often . Any suggestion in this regard ?
> >

> I have seen the pool locked in this case (using a debugger):
>
>     1.  A pbuf_* function is called.  It locks the pool.
>     2.  The pbuf_ function indirectly results in a call to sys_sem_* or
> similar.
>     3.  The sem function results in a call to a timeout function.
>     4.  Finally, the timeout function calls a pbuf_* function.  The pool
> is still locked from step 1.

The pbuf_pool_free_lock and pbuf_pool_alloc_lock are only used by
pbuf_pool_alloc and pbuf_refresh.  Neither call a sys_sem_* or similar
while locked, so I don't think this can be right.

Adam and I discussed this some time ago.  See the thread beginning with
http://www.sics.se/~adam/lwip/maillist/msg00600.html

I implemented the suggestion, but it didn't seem to alleviate the problem,
and it was a bit of a hack so I backed it out.  Instead, I've decided to
re-engineer the structure of the stack to avoid the problem, but this is
unlikely to be a change that most people would be interested in as it'll
be fairly major, and specific to my network.

Kieran

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