[lwip-devel] Problem with netconn_write()

[Mountifield, Tony]

Hi All,

I've just run into a problem with netconn_write() and its interaction with 
sent_tcp().

Having sped up my execution by enabling the CPU cache (this is new hardware), I 
started finding the underlying OS would occasionally hit a fatal error due to 
trying to signal a deleted semaphore. This was always in sent_tcp(), in the 
following code:

  if (conn != NULL && conn->sem != SYS_SEM_NULL) {
    sys_sem_signal(conn->sem);
  }

The thread was being pre-empted after testing conn->sem, so that by the time my 
implementation of sys_sem_signal() called the OS, another thread had already 
come along and deleted the semaphore.

I deduced that the culprit was netconn_write() in api_msg.c: it creates a 
semaphore on entry, if it doesn't already exist, whic is fair enough. However, 
it deletes it again just before it exits. So for every write a semaphore is 
getting created and then deleted. Surely it would be better to leave the 
semaphore in existence, and  free it in netconn_close().

In any case, assuming sys_sem_signal() and sys_sem_free() can't block, there 
should perhaps be some exclusion to make sure that a test of conn->sem and its 
subsequent use are indivisble, for semaphores that can be dynamically deleted.

Comments?

Tony
-- 
Tony Mountifield
Contractor @ Tandberg TV
Strategic Park, ext 3390
Tel: 023 8057 3390


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