Aggressive GC? Was Re: Untimely garbage collection

Scott Lystig Fritchie <>
Sat Jul 13 22:00:46 CEST 2002


While hacking drivers, I've come across a GC question, similar to
Shawn's, that I cannot answer.  

Consider this code snippet:

	iolist2binary(B) when binary(B) ->
	    {B, size(B)};
	iolist2binary(L) when List(L) ->
	    B = list_to_binary(L),
	    {B, size(B)}.
	
	foofoo(Port, IOList) when port(Port) ->
	    {IOListBinary, IOListBinaryLen} = iolist2binary(IOList),
	    C = [ <<?S1_FOOFOO, IOListBinaryLen:32/integer>>, IOListBinary],
	    erlang:port_command(Port, C),
	    %% Vulnerability window begins here?
	    get_port_reply(Port).
	
	get_port_reply(Port) ->
	    receive
	        {Port, ok} -> ok;
	        [...]

Assume:

	1. The driver caches the pointer to IOListBinary's data
	buffer.  This permits the driver to avoid making a copy of
	that data buffer: it can access the binary's data buffer
	directly.

	2. The driver is implemented asynchonously, using a separate
	Pthread.  Thus the VM can do other work while the driver's
	Pthread takes an arbitrary amount of time to do whatever it
	does.

My question: Is it possible for the VM's GC to decrement
IOListBinary's refcount in the time between execution of the
port_command() and the receive?  If the VM's GC were extremely
aggressive(*), then if a GC happened at "Vulnerability window begins
here?", then:

	a. IOListBinary's refcount could go to zero: if the original
	IOList were a deep byte list, then foofoo() is the only thing
	that knows about IOListBinary, so its refcount would drop to
	zero.

	b. Disaster!  The driver's cached pointer to IOListBinary's
	data buffer is invalid.  The driver is executing independently
	of the VM's Pthread, so there's no guarantee that the driver
	will use the pointer before the pointer becomes invalid.

-Scott

(*) If the GC system can figure out that a binding has not yet gone
out of scope yet *but* that binding will never be used again, then
... I'm in trouble.



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