[issue9787] Release the TLS lock during allocations

Kristján Valur Jónsson report at bugs.python.org
Mon Sep 13 05:24:29 CEST 2010

Kristján Valur Jónsson <kristjan at ccpgames.com> added the comment:

You may find this hard to believe, but we do in fact embed python into other applications.  In this case, it is UnrealEngine, to drive a complex, console based game.  Yes, embedding python is much harder than it need be and I'll submit some patches to make that easier someday, but that's not the point of this.

appMalloc, is in this case, the canonical memory allocator in UnrealEngine.  But it could be any other memory allocator so that is beside the point.

The problem at hand, however, is this memory allocator _may_ have to inquire about the state of Python.  It would do this, for example, to gather statistics about Python's memory use.  This is critically important when developing console based applications, where every Kilobyte counts.  Embedding python sometimes requires the replacement of  the standard libc malloc with something else.

What appMalloc is doing, in this case, is for every allocation, to get the python TLS pointer.  There is nothing wrong with this, this is a GIL free operation, and it will return either NULL or the current TLS.  And it works, except, when appMalloc (through python's malloc) is being invoked from the TLS entry creation mechanism itself.  Then it deadlocks.

Now, regardless of the above, surely it is an improvement in general if we make tighter use of the TLS lock.  It's not a good idea to hold this lock across malloc calls if we can avoid it.  The patch is harmless, might even be an improvement, so why object to it?  It removes yet another "gotcha" that embedders, or those replacing malloc, (or instrumenting python's malloc use) have to face.




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