Re: [lwip-users] LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT

[Giuseppe Modugno]

Thanks Simon. I want to summarize here all the possibilities with 
NO_SYS=1, please confirm if I'm right.

** MEM_USE_POOLS=0, MEM_LIBC_MALLOC=0
lwip internal dynamic memory allocator from a static array (implemented 
in mem.c)
To use pbuf_free() in ISR (and main loop), we need to set 
LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT *and* define 
SYS_ARCH_PROTECT/UNPROTECT as macros that disable/enable interrupts.
Of course, sys_mutex_lock() and similar functions can be void, because 
there's only one thread (mainloop) in NO_SYS=1.

** MEM_USE_POOLS=1, MEM_LIBC_MALLOC=0
Lwip memory pools used as dynamic memory allocator (mem.c and memp.c)
As you wrote, memory pools implementation in memp.c is reentrant, so 
pub_free can be called safely from ISR, even without 
LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT.
What I don't understand if SYS_ARCH_PROTECT (that is used in memp) 
should disable interrupts or this isn't needed.
If yes, this solution doesn't solve the problem of disabling interrupts 
frequently.

** MEM_USE_POOLS=1, MEM_LIBC_MALLOC=1
Lwip malloc/free calls libc malloc/free
In my case, I'm using newlib-nano. Even in this case, from newlib 
documentation[1], I need to implement __malloc_lock/unlock that must 
disable interrupts again.


If all is correct, there isn't a "standard" solution to avoid disabling 
interrupts when pbuf_free is called in ISR context (always for NO_SYS=1).
Anyway I think it shouldn't be so difficult to write a 
pbuf_free_callback for NO_SYS=1 that pushes the pbuf to be freed in a 
simple FIFO implementation.
In the mainloop, a pbuf_free_task can be continuously called. This 
function pops a pbuf from the FIFO and calls pbuf_free on it.


[1] https://sourceware.org/newlib/libc.html#g_t_005f_005fmalloc_005flock



Il 11/10/2022 06:25, Simon Goldschmidt ha scritto:
> Am 10. Oktober 2022 18:32:15 MESZ schrieb Giuseppe Modugno 
> <giuseppe.modugno.loqed@gmail.com>:
> > I'm sorry to post again.
> >
> > I just understood LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEX is used in mem.c for 
> > MEM_USE_POOLS=0 and MEM_LIBC_MALLOC=0 (default lwip heap management from a 
> > statically allocated buffer).
> >
> > What about the reentrancy of memory pools? What happens when MEM_USE_POOLS=1 
> > and pbuf_free is called in ISR, with or without 
> > LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT? It seems memory pools free is safe to 
> > be used from ISR, because I don't see any protection for them (differently of 
> > lwip default dynamic allocator).
> Yes, it is.
>
> Regards,
> Simon
>
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