[lwip-devel] [bug #21433] Calling mem_free/pbuf_free from interrupt context isn't safe

[Simon Goldschmidt]

Follow-up Comment #12, bug #21433 (project lwip):

> plug_holes isn't a problem

Of course it's not, since there is no loop in it!

[Kieran wrote:]
> I have to admit I'm a bit uneasy about increasing the amount of > stuff
that we support from interrupt [->bad code]

[Jonathan wrote:]
> I would have guessed (possibly wrongly) that most ethernet
> driver systems do operate at thread level because the amount
> of processing

For RX, I agree; for TX enqueuing also. But freeing ref'ed pbufs when they
are actually sent can be done quite fast in interrupt context, I think
(nothing to be copied), and I don't see bad coding here.

> [if] SYS_ARCH_PROTECT is OK then that would be a relatively 
> simple change I think.

If it's configurable it would be OK with me but in general it leads to very
long periods of time where the interrupts are disabled (which _is_ bad code to
me; after all it's nearly the same as a very long running ISR). But since this
seems the only nice solution for both NO_SYS settings, I'm OK with it as long
as

a) using semaphore is the default
b) saying 'pbuf_free is not supported from interrupt level' is our default
and
c) pointing users to documentation that explicitly tells them to enable the
SYS_ARCH in mem.c so that they can use pbuf_free from interrupt context

> Either way, I think this is best left till after 1.3.0
> otherwise we'll never get it out. 

But then again, maybe using tcpip_callback isn't so bad after all? :-)

But honestly: we can just introduce an option to use SYS_ARCH_PROTECT, that's
easily done before 1.3.0!

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