> So, is it really forbidden to release the global lock in a noalloc function?

Yes.  Actually, it is forbidden to call any function of the OCaml
runtime system from a noalloc function.

Explanation: ocamlopt-generated code caches in registers some global
variables of importance to the OCaml runtime system, such as the
current allocation pointer.

When calling a regular (no-"noalloc") C function from OCaml, these
global variables are updated with the cached values so that everything
goes well if the C function allocates, triggers a GC, or releases the
global lock (enabling a context switch).

This updating is skipped when the C function has been declared
"noalloc" -- this is why calls to "noalloc" functions are slightly
faster.  The downside is that the runtime system is not in a
functioning state while within a "noalloc" C function, and must
therefore not be invoked.

The cost of updating global variables is small, so "noalloc" makes
sense only for short-running C functions (say, < 100 instructions) like
those from the math library (sin, cos, etc).  If the C function makes
significant work (1000 instructions or more), just play it safe and
don't declare it "noalloc".

Hope this helps,

- Xavier Leroy