From f1b71690c260e9235375cbd40a7a15df1f57ca3d Mon Sep 17 00:00:00 2001
From: Alex Boten <223565+codeboten@users.noreply.github.com>
Date: Thu, 18 Jun 2026 13:28:15 -0700
Subject: [PATCH 1/2] opentelemetry-sdk: revert RLock back to Lock

With the changes in clean_attributes, the original issue that required the reentrant lock in the first place is no longer an issue. Added a test that was run with the old code to validate that the regression does not occur.

Signed-off-by: Alex Boten <223565+codeboten@users.noreply.github.com>
---
 .../src/opentelemetry/attributes/__init__.py  |  2 +-
 .../tests/attributes/test_attributes.py       | 44 +++++++++++++++++++
 2 files changed, 45 insertions(+), 1 deletion(-)

diff --git a/opentelemetry-api/src/opentelemetry/attributes/__init__.py b/opentelemetry-api/src/opentelemetry/attributes/__init__.py
index 15519ce0c2c..9c1b83b91e6 100644
--- a/opentelemetry-api/src/opentelemetry/attributes/__init__.py
+++ b/opentelemetry-api/src/opentelemetry/attributes/__init__.py
@@ -262,7 +262,7 @@ def __init__(
             MutableMapping[str, types.AnyValue]
             | OrderedDict[str, types.AnyValue]
         ) = {}
-        self._lock = threading.RLock()
+        self._lock = threading.Lock()
         if attributes:
             for key, value in attributes.items():
                 self[key] = value
diff --git a/opentelemetry-api/tests/attributes/test_attributes.py b/opentelemetry-api/tests/attributes/test_attributes.py
index fb6259f8c47..6313b69386e 100644
--- a/opentelemetry-api/tests/attributes/test_attributes.py
+++ b/opentelemetry-api/tests/attributes/test_attributes.py
@@ -4,6 +4,8 @@
 # type: ignore
 
 import copy
+import logging
+import threading
 import unittest
 import unittest.mock
 from collections.abc import MutableSequence
@@ -283,6 +285,48 @@ def test_locking(self):
         for num in range(100):
             self.assertEqual(bdict[str(num)], num)
 
+    def test_no_deadlock_on_reentrant_logging(self):
+        """Regression test for #3858.
+
+        The deadlock scenario: a logging handler intercepts the warning
+        emitted by _clean_attribute for an invalid value and calls __setitem__
+        on the same BoundedAttributes instance from the same thread.
+        With _clean_attribute called inside the lock this caused a deadlock.
+        With _clean_attribute called before the lock is acquired, no deadlock
+        occurs.
+        """
+        bdict = BoundedAttributes(immutable=False)
+
+        class ReentrantHandler(logging.Handler):
+            def emit(self, _record):
+                # Simulates Sentry intercepting the OTel warning and writing
+                # back into the same BoundedAttributes on the same thread.
+                bdict["reentrant.key"] = "set_by_handler"
+
+        otel_logger = logging.getLogger("opentelemetry.attributes")
+        handler = ReentrantHandler()
+        otel_logger.addHandler(handler)
+        try:
+            completed = threading.Event()
+
+            def run():
+                # None is an invalid attribute value and triggers _logger.warning
+                # in _clean_attribute, which fires the ReentrantHandler above.
+                bdict["trigger.key"] = None
+                completed.set()
+
+            t = threading.Thread(target=run, daemon=True)
+            t.start()
+            t.join(timeout=2.0)
+
+            self.assertTrue(
+                completed.is_set(),
+                "Deadlock detected: __setitem__ did not complete within 2s",
+            )
+            self.assertEqual(bdict.get("reentrant.key"), "set_by_handler")
+        finally:
+            otel_logger.removeHandler(handler)
+
     # pylint: disable=no-self-use
     def test_extended_attributes(self):
         bdict = BoundedAttributes(extended_attributes=True, immutable=False)

From 8f7289bff918cbcef7ffceacc14730c23f59580b Mon Sep 17 00:00:00 2001
From: Alex Boten <223565+codeboten@users.noreply.github.com>
Date: Thu, 18 Jun 2026 13:34:36 -0700
Subject: [PATCH 2/2] changelog

Signed-off-by: Alex Boten <223565+codeboten@users.noreply.github.com>
---
 .changelog/5329.changed | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 .changelog/5329.changed

diff --git a/.changelog/5329.changed b/.changelog/5329.changed
new file mode 100644
index 00000000000..545518c6f3e
--- /dev/null
+++ b/.changelog/5329.changed
@@ -0,0 +1 @@
+opentelemetry-sdk: revert RLock back to Lock
\ No newline at end of file