[Click] Race condition behavior with single thread Click Linux module

[Paine, Thomas Asa]

	I was under the assumption that click running in kernel time, by
default, ran under a single thread.  We have run into an issue when we
have a read/write handler on an element clearing a hashmap that is a
member of that element.  If the map is cleared through a function called
from the static handler (hashmap.clear()), we kernel panic (the map is
updated with every packet passing through the element and at high packet
rates).  
	If the handler calls a helper function that uses a timer within
the element (timer.schedule_now()), this does not happen.  This suggests
to me there is a race condition on the element's state and that there is
actually more than one thread running.  Am I correct here, and if so
should I assume locks are needed for thread safety with regards to
proc's view into an element and its static handlers?


In write_handler() below I have commented what causes the panic and what
does not.



int FilterTables::initialize(ErrorHandler *errh){
  timer.initialize(this);
  timer.schedule_now();
}

Packet* FilterTables::simple_action(Packet *p){ .
.
  _byteMap->insert(ip, _byteMap->find(ip)+len); .
.
}

void FilterTables::add_handlers(){
.
.
  // Write Handlers
  add_write_handler("reset", write_handler, (void *)H_RESET); .
.
}


int FilterTables::write_handler(const String &in_str, Element *e, void
*thunk, ErrorHandler *errh){
  FilterTables *c = (FilterTables *)e;
.
.
  switch ((intptr_t)thunk) {
   case H_RESET:
    // this will cause a panic, but not always the first time called
(which again acts like a race condition)
    c->reset();
    // but this will not cause a panic and clears the map
    timer.schedule_now();	
    return 0;
.
.
}

void FilterTables::reset(){
  _byteMap->clear();
}


void FilterTables::run_timer(){
  reset();
}



Thanks,
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
   Thomas Paine (paineta at uwec.edu) 
   University of Wisconsin - Eau Claire 
   garbage foo(garbage g){return(g);}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~