Re: [lwip-users] LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT

[Giuseppe Modugno]

Il 12/10/2022 21:11, goldsimon@gmx.de ha scritto:
> Am 11.10.2022 um 17:11 schrieb Giuseppe Modugno:
> > Another thing I can't understand is why the code around
> > LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT is needed if:
> >
> >   * NO_SYS=1
> >   * MEM_USE_POOLS=0
> >   * MEM_LIBC_MALLOC=0
> >   * SYS_LIGHTWEIGHT_PROT=1
> >   * sys_arch_protect() simply disables interrupts
> >
> > In mem_malloc there's a point where LWIP_MEM_ALLOC_PROTECT is called,
> > before the "scan through the heap searching for a free block". Why
> > during the scan, for each loop, UNPROTECT/PROTECT are called? The
> > comment says /* allow mem_free or mem_trim to run */.
> >
> > Maybe this is done because the scan could take a long time to finish
> > (for big and fragmented heap) and having interrupts disabled for so long
> > time is not good? So the interrupts are enabled and disabled for each
> > loop to give them opportunity to run?
>
> Exactly:
> - Heap allocation takes a mutex as it should not block interrupts while
> scanning for memory (takes too long).
> - Heap free is fast as it does not need to scan.
>
> Therefore, heap allocation uses double protection: mutex to block
> against other allocations, PROTECT/UNPROTECT to protect against
> concurrent free.

Ok, thank you for the answer.

Do you think it's possible to implement a pbuf_free_callback() in 
NO_SYS=1 that can be called in ISR instead instead of pbuf_free()?

The goal is to avoid disabling interrupts at all setting 
SYS_LIGHTWEIGHT_PROT=0 to disable PROTECT/UNPROTECT mechanism.

The only lwip function that NXP code uses in ISR is pbuf_free() (that 
needs to be protected with disabling interrupts and 
LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT), so I'm searching an alternative 
way to avoid this.