RE: GC causes unexpected reentrancy

["Kevin Baca" <lualist@...>]

How about turning GC on and off as needed?

Requires source modification.  See the luaC_checkGC macro.

-Kevin


> 
> Some background: I'm working on a multi-threaded (as in 
> system threads) application in which multiple threads can all 
> access the same Lua universe through multiple lua_States. 
> They end up serializing at the Lua API because of the way 
> lua_lock works, but that's not important.
> 
> Now, consider the case where I've got userdata that needs to 
> store Lua references of some sort. In my case, it's that I've 
> got reference counts on userdata items for non-Lua references 
> and any items with non-zero reference counts have to be 
> entered into a Lua table so that they don't get garbage 
> collected. I believe, however, that the same problems would 
> arise if the userdata items held Lua refs.
> 
> Because the application is multi-threaded, when I update the 
> Lua data structures (userdata retention table or a ref 
> table), I need to bracket those operations with mutex locks.
> 
> So, now consider the following path....
> 
> * Retain an object (or release it)
>     * Locks the retention table mutex
>         * Calls to Lua to modify the table
>             * Lua invokes the GC
>                 * The GC invokes a userdata GC method
>                     * Userdata GC method releases an object
>                         * Attempt to lock the retention table mutex
> 
> Deadlock.
> 
> For my particular usage, I might be able to resolve my 
> problem by using a recursive mutex for the retention table, 
> but that gets a bit delicate. For the ref case, I've looked 
> at luaL_ref and it makes multiple Lua API calls and it 
> doesn't seem wise to me to re-enter any of that code so a 
> recursive mutex wouldn't help.
> 
> Any suggestions?
> 
> My current thought is that I should look at the sweep phase 
> of the garbage collector and work on making the portion that 
> calls the GC methods for userdata run in a separate thread. 
> That, however, is going to require changing the Lua sources.
> 
> Mark
>