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This section discusses compiling files for the first time. For reloading, see section 4.3.2.
In older versions, compilation was thread-safe due to a global lock in load_files/2 and the code dealing with autoloading (see section 2.13). Besides unnecessary stalling when multiple threads trap unrelated undefined predicates, this easily leads to deadlocks, notably if threads are started from an initialization/1 directive.51Although such goals are started after loading the file in which they appear, the calling thread is still likely to hold the `load' lock because it is compiling the file from which the file holding the directive is loaded.
Starting with version 5.11.27, the autoloader is no longer locked and multiple threads can compile files concurrently. This requires special precautions only if multiple threads wish to load the same file at the same time. Therefore, load_files/2 checks automatically whether some other thread is already loading the file. If not, it starts loading the file. If another thread is already loading the file, the thread blocks until the other thread finishes loading the file. After waiting, and if the file is a module file, it will make the public predicates available.
Note that this schema does not prevent deadlocks under all situations. Consider two mutually dependent (see section 18.104.22.168) module files A and B, where thread 1 starts loading A and thread 2 starts loading B at the same time. Both threads will deadlock when trying to load the used module.
The current implementation does not detect such cases and the involved threads will freeze. This problem can be avoided if a mutually dependent collection of files is always loaded from the same start file.