Re: include LuaThread to Lua 5.1.3

["Javier Guerra" <javier@...>]

On Thu, Jul 3, 2008 at 10:07 PM, GaoXianchao <gxcmaillist@gmail.com> wrote:
> Hi Javier Guerra Giraldez ,

hi (just Javier, please :-)

> I have Another question on luathread:

Diego is the best one to answer the hairy details... i'm just an
interested observer of his code.

> If I call a c function which do something like calculating SHA1 hash of a
> large file in thread B,  when I call another c function in thread A, it will
> be blocked or not? I see lots of  "lua_lock(L)" in lapi.c.

plain Lua already has all these lua_lock()/lua_unlock() in place, but
these are macros that expand to nothing.  LuaThreads adds the
infrastructure to make them useful.  unfortunately, it's a single lock
for the whole shared LuaSpace; what is called GIL in other languages
(Global Interpreter Lock, same issue at least in Python, and probably
Ruby... Perl too? anybody knows?)

anyway, see ldo.c: where the call to C is done in luaD_precall().
there you can see the actual call is preceded by a lua_unlock(L), and
immediately followed by lua_lock(L).

IOW: the GIL sequentialises Lua, but when you call C, the lock is
released and other threads are free to run.

all the lua_lock()/lua_unlock() 's in lapi.c makes it safe to use Lua
C API from any thread, the GIL will be acquired as needed before
meddling with the LuaState.

-- 
Javier