Jerome Vuarand wrote:
2009/12/2 Matt 'Matic' (Lua) <lua@photon.me.uk>:
...
As you both have said, it would on the surface appear as though I had two
threads accessing one lua_State, or some other C/C++ corruption of Lua.
However, I've put a huge amount of code to prove that wasn't the case (just
doubting myself!!). Everything appears correct and coherent.
My view is that the Lua VM is making assumptions about the L->base value.
Generally, L->base doesn't change, so it holds a local copy to avoid
indirection and speed up the LVM. In some opcodes, the VM knows that L->base
could or is definitely going to change, so the call is wrapped up in the
"Protect" macro which reassigned the local copy of base after completion.
However, it would appear that there are 6 (IIRC) op codes that call dojump
outside of the context of "Protect" and assume that L->base is not going to
change. If you have one Lua "Universe", one lua_State per OS thread then
that will be ok.
Once you add OS-level threading and have a single Lua "Universe" - even if
you use lua_newthread and strictly keep each lua_State in an OS thread -
that assumption is no longer valid.
Consequently, I have changed the "dojump" macro in lvm.c to now be:
#define dojump(L,pc,i) { (pc) += (i); luai_threadyield(); base =
L->base; }
Now, I know that some "dojump" calls are also wrapped inside "Protect" and
therefore the "base = L->base" is going to be duplicated in those cases. Of
course, my optimising compiler removes the redundancy.
Guess what - the problem has gone away and Lua is not failing its assertions
anymore (and my Lua code isn't running off the rails)!!
Javier - I reckon you adding the extra lock/unlock inside your C routines is
probably greatly minimising the issue because you are reducing its
probability, but you may well find it's the same one that I have and can be
resolved completely with the dojump patch.
Any thoughts or comments??
Do you have a patch of the modifications available ?
lvm.c (version 2.63.1.3 2007/12/28 15:32:23), line 360:
#define dojump(L,pc,i) {(pc) += (i);
luai_threadyield(L);}
Change to:
#define dojump(L,pc,i) {(pc) += (i);
luai_threadyield(L); base = L->base;}
That's all there is to it :-)
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