Re: [lwip-users] LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT

[address@hidden]

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.

Regards,
Simon