[Vm-dev] Re: Problem with alien callbacks

[Eliot Miranda]

On Thu, Mar 31, 2011 at 6:11 AM, Javier Pimás <elpochodelagente at gmail.com>wrote:

> Hi, we are having a problem with callbacks in alien and we would like to
> see if we are doing something wrong or if it is a bug in the implementation
> (for the standard old vm).
>
> We are receiving the callback just in the middle of a primitiveExternalCall
> (actually to a function that will fail because the plugin is not present,
> but i don't think that's important). We pinned it to occur always in the
> same line, which is
>
> longAtput((lit + (BASE_HEADER_SIZE)) + (2 << (SHIFT_FOR_WORD)), ConstZero);
>
> of primitiveExternalCall. When the callback occurs, the thunkEntry is
> called, which if we understand correctly, saves the active context and runs
> the interpreter by calling sendInvokeCallbackStackRegistersJmpbuf. The
> problem is that things like messageSelector and lkupClass, which are global
> variables are not saved while saving the context, and when the callback
> returns, the last line of primitiveExternalCall,
>
> rewriteMethodCacheSelclassprimIndex(messageSelector, lkupClass, 0);
>
> puts a 0 in the wrong place. Also, probably as las message sent
> was primReturnFromContext:through: (because we just returned from the
> context), we get a primitiveFailed, but not for the original called function
> but for primReturnFromContext:through:.
>
> What do you think? are we missing something?
>

Hmmm, looking at it I think you must be taking a callback before the
external call occurs.  Here's how the code reads in Cog:


...

addr := self ioLoadExternalFunction: functionName + BaseHeaderSize
OfLength: functionLength
FromModule: moduleName + BaseHeaderSize
OfLength: moduleLength.
addr = 0
ifTrue: [index := -1]
ifFalse: ["add the function to the external primitive table"
index := self addToExternalPrimitiveTable: addr].

"Store the index (or -1 if failure) back in the literal"
objectMemory storePointerUnchecked: 3 ofObject: lit withValue: (objectMemory
integerObjectOf: index).

"If the function has been successfully loaded cache and call it"
index >= 0
ifTrue:
[self rewriteMethodCacheEntryForExternalPrimitiveToFunction: (self cCode:
[addr] inSmalltalk: [1000 + index]).
self callExternalPrimitive: addr]
ifFalse: ["Otherwise void the primitive function and fail"
self rewriteMethodCacheEntryForExternalPrimitiveToFunction: 0.
^self primitiveFailFor: PrimErrNotFound]

So the rewrite to zero (self
rewriteMethodCacheEntryForExternalPrimitiveToFunction: 0) isn't done if no
callout is made.  Where is your callback comming from?  Looks like its
comming from the internals of things like ioLoadExternalFunction...

It is hard to save and restore all the VM state around a callback.  There's
too much of it in the current VM design.  Take a look
at rewriteMethodCacheEntryForExternalPrimitiveToFunction:.  It is written to
be fast, using lastMethodCacheProbeWrite to avoid work in rewriting the
cache entry if the module and/or function load fails.  That's state one
doesn't want to have to save and restore around callbacks along with
lkupClass, messageSelector.  primitiveFunctionPointer, newMethod,
framePointer, instructionPointer and stackPointer are already a lot.  Tthis
needs more thought.


> Regards,
>             Javier.
>
>
> --
> Javier Pimás
> Ciudad de Buenos Aires
>
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