From 4223acc8445f62d7cb33ef527ed395d1ea9fc4f8 Mon Sep 17 00:00:00 2001
From: Chris Fonnesbeck <fonnesbeck@gmail.com>
Date: Tue, 27 May 2025 21:37:14 -0500
Subject: [PATCH 1/4] Set multiprocessing to spawn for Linux

---
 pymc/sampling/mcmc.py | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/pymc/sampling/mcmc.py b/pymc/sampling/mcmc.py
index f2dfa6e9c2..fae0107f9e 100644
--- a/pymc/sampling/mcmc.py
+++ b/pymc/sampling/mcmc.py
@@ -17,6 +17,7 @@
 import contextlib
 import logging
 import pickle
+import platform
 import sys
 import time
 import warnings
@@ -78,6 +79,11 @@
 )
 from pymc.vartypes import discrete_types
 
+if platform.system() == "linux":
+    import multiprocessing
+
+    multiprocessing.set_start_method("spawn", force=True)
+
 try:
     from zarr.storage import MemoryStore
 except ImportError:

From 59b3ad35ac1aa7670cfd4a58f318cba4d392eea5 Mon Sep 17 00:00:00 2001
From: Chris Fonnesbeck <fonnesbeck@gmail.com>
Date: Tue, 27 May 2025 21:45:21 -0500
Subject: [PATCH 2/4] Added code comment

---
 pymc/sampling/mcmc.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/pymc/sampling/mcmc.py b/pymc/sampling/mcmc.py
index fae0107f9e..c99f89003c 100644
--- a/pymc/sampling/mcmc.py
+++ b/pymc/sampling/mcmc.py
@@ -80,6 +80,7 @@
 from pymc.vartypes import discrete_types
 
 if platform.system() == "linux":
+    # Threads are not fork-safe on Linux, so we need to use spawn
     import multiprocessing
 
     multiprocessing.set_start_method("spawn", force=True)

From ff251340152c2d7440cb2b5a64211a2e320bf047 Mon Sep 17 00:00:00 2001
From: Chris Fonnesbeck <fonnesbeck@gmail.com>
Date: Tue, 27 May 2025 22:10:36 -0500
Subject: [PATCH 3/4] Moved linux mp setting to parallel.py with the other
 platform settings

---
 pymc/sampling/mcmc.py     |  7 -------
 pymc/sampling/parallel.py | 22 +++++++++++++++-------
 2 files changed, 15 insertions(+), 14 deletions(-)

diff --git a/pymc/sampling/mcmc.py b/pymc/sampling/mcmc.py
index c99f89003c..f2dfa6e9c2 100644
--- a/pymc/sampling/mcmc.py
+++ b/pymc/sampling/mcmc.py
@@ -17,7 +17,6 @@
 import contextlib
 import logging
 import pickle
-import platform
 import sys
 import time
 import warnings
@@ -79,12 +78,6 @@
 )
 from pymc.vartypes import discrete_types
 
-if platform.system() == "linux":
-    # Threads are not fork-safe on Linux, so we need to use spawn
-    import multiprocessing
-
-    multiprocessing.set_start_method("spawn", force=True)
-
 try:
     from zarr.storage import MemoryStore
 except ImportError:
diff --git a/pymc/sampling/parallel.py b/pymc/sampling/parallel.py
index af2106ce6f..bac2103404 100644
--- a/pymc/sampling/parallel.py
+++ b/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 when using libraries like OpenBLAS
+                    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)
 

From 8b6f9e67becf768354f73abc85c704c24fecad5f Mon Sep 17 00:00:00 2001
From: Chris Fonnesbeck <fonnesbeck@gmail.com>
Date: Tue, 27 May 2025 22:13:45 -0500
Subject: [PATCH 4/4] updated code comment

---
 pymc/sampling/parallel.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pymc/sampling/parallel.py b/pymc/sampling/parallel.py
index bac2103404..292bb4d982 100644
--- a/pymc/sampling/parallel.py
+++ b/pymc/sampling/parallel.py
@@ -448,7 +448,7 @@ def __init__(
                     else:
                         mp_ctx = "forkserver"
                 elif platform.system() == "Linux":
-                    # Threads are not fork-safe on Linux when using libraries like OpenBLAS
+                    # Threads are not fork-safe on Linux
                     mp_ctx = "spawn"
                     logger.debug(
                         "mp_ctx is set to 'spawn' for Linux to ensure thread safety. "