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Re: [lwip-users] Reentrency problem within mem.c module

From: EVS Hardware Dpt
Subject: Re: [lwip-users] Reentrency problem within mem.c module
Date: Thu, 25 Oct 2007 18:54:38 +0200

pbuf_free_safe() would be enough in this situation, but since memp_free
is the problem (no pbuf_free()),
I think memp_free_safe() would be better since it is more generic.

mem_free_safe could do the trick but only when paired with a mem_malloc_safe. Or you have another idea with mem_free_safe.

I like your idea of putting a list of must_be_freed buffer.

We could have a lock variable that is set in mem_malloc

void mem_malloc()
   //Acquire Lock
   mem_lock = 1;

   //Do all the allocating stuff

   //Release Lock
   mem_lock = 0;

in mem_free()
   if(mem_lock == 1)
      //Put buffer on the must_be_freed list for later processing (mem_free_list)
      //Continue the free process

     //And also free other item in the must_be_freed list

We could then process list in another portion of the stack and why not in mem_malloc.

As a side note, this kind of functionality (list access) could be achieved with some Atomic primitives (InterlockedCompareExchange, AtomicCompareExchange). I our code we use often this kind of functions. This could be interesting to provide some macro, that could be redefined to meet each platform.

A basic implementation (compatible with all platforms) would be
typedef u32_t uDef_t; //Default architecture unsigned value (here, 32 bits processor => 32 bits unsigned value)

uDef_t AtomicCompareExchange(uDef_t* Address, uDef_t Comparand, uDef_t NewVal)
   uDef_t OldValue;

   old_level = SYS_ARCH_PROTECT();
   if((OldValue = *Address) == Comparand){
      *Address = NewVal;     
   return OldValue;

In a win32 system this would be the InterlockedCompareExchange function, in a PowerPC platform this would be a function using atomic primitives functionnalities.


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