Set multiprocessing to spawn for Linux

pymc/sampling/parallel.py

@@ -437,15 +437,23 @@ def __init__(
         if mp_ctx is None or isinstance(mp_ctx, str):
             # Closes issue https://github.com/pymc-devs/pymc/issues/3849
             # Related issue https://github.com/pymc-devs/pymc/issues/5339
-            if mp_ctx is None and platform.system() == "Darwin":
-                if platform.processor() == "arm":
-                    mp_ctx = "fork"
+            if mp_ctx is None:
+                if platform.system() == "Darwin":
+                    if platform.processor() == "arm":
+                        mp_ctx = "fork"
+                        logger.debug(
+                            "mp_ctx is set to 'fork' for MacOS with ARM architecture. "
+                            + "This might cause unexpected behavior with JAX, which is inherently multithreaded."
+                        )
+                    else:
+                        mp_ctx = "forkserver"
+                elif platform.system() == "Linux":
+                    # Threads are not fork-safe on Linux
+                    mp_ctx = "spawn"
                     logger.debug(
-                        "mp_ctx is set to 'fork' for MacOS with ARM architecture. "
-                        + "This might cause unexpected behavior with JAX, which is inherently multithreaded."
+                        "mp_ctx is set to 'spawn' for Linux to ensure thread safety. "
+                        + "This is required when using multithreaded numerical libraries."
                     )
-                else:
-                    mp_ctx = "forkserver"
 
             mp_ctx = multiprocessing.get_context(mp_ctx)