From 1a821390d11d3bc66e6858f27dceeef67e14078d Mon Sep 17 00:00:00 2001
From: Aleksand Malikov <schn27@gmail.com>
Date: Mon, 23 Aug 2021 17:27:48 +0300
Subject: [PATCH] fix race condition in List_insert

Revert mx parameter and mutex lock while operating the list.
Mutex was removed with 94d24160 informing that:
'Use InterlockedCompareExchangePointer to ensure race safeness
without using a mutex.'

But it does not.

Calling pthread_mutex_init and pthread_mutex_destroy from two or
more threads occasionally leads to hang in pthread_mutex_destroy.

To not change the behaviour of other cases where List_insert was called,
List_insert_nolock is added.
---
 winsup/cygwin/thread.cc |  8 ++++----
 winsup/cygwin/thread.h  | 17 +++++++++++++++--
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/winsup/cygwin/thread.cc b/winsup/cygwin/thread.cc
index 4d0ea274f..7c6a919c0 100644
--- a/winsup/cygwin/thread.cc
+++ b/winsup/cygwin/thread.cc
@@ -1595,7 +1595,7 @@ pthread_rwlock::add_reader ()
 {
   RWLOCK_READER *rd = new RWLOCK_READER;
   if (rd)
-    List_insert (readers, rd);
+    List_insert_nolock (readers, rd);
   return rd;
 }
 
@@ -2165,7 +2165,7 @@ pthread::atfork (void (*prepare)(void), void (*parent)(void), void (*child)(void
   if (prepcb)
   {
     prepcb->cb = prepare;
-    List_insert (MT_INTERFACE->pthread_prepare, prepcb);
+    List_insert_nolock (MT_INTERFACE->pthread_prepare, prepcb);
   }
   if (parentcb)
   {
@@ -2174,7 +2174,7 @@ pthread::atfork (void (*prepare)(void), void (*parent)(void), void (*child)(void
     while (*t)
       t = &(*t)->next;
     /* t = pointer to last next in the list */
-    List_insert (*t, parentcb);
+    List_insert_nolock (*t, parentcb);
   }
   if (childcb)
   {
@@ -2183,7 +2183,7 @@ pthread::atfork (void (*prepare)(void), void (*parent)(void), void (*child)(void
     while (*t)
       t = &(*t)->next;
     /* t = pointer to last next in the list */
-    List_insert (*t, childcb);
+    List_insert_nolock (*t, childcb);
   }
   return 0;
 }
diff --git a/winsup/cygwin/thread.h b/winsup/cygwin/thread.h
index 6b699ccb6..ddb2d7dbf 100644
--- a/winsup/cygwin/thread.h
+++ b/winsup/cygwin/thread.h
@@ -111,7 +111,20 @@ typedef enum
 } verifyable_object_state;
 
 template <class list_node> inline void
-List_insert (list_node *&head, list_node *node)
+List_insert (fast_mutex &mx, list_node *&head, list_node *node)
+{
+  if (!node)
+    return;
+  mx.lock ();
+  do
+    node->next = head;
+  while (InterlockedCompareExchangePointer ((PVOID volatile *) &head,
+					    node, node->next) != node->next);
+  mx.unlock ();
+}
+
+template <class list_node> inline void
+List_insert_nolock (list_node *&head, list_node *node)
 {
   if (!node)
     return;
@@ -163,7 +176,7 @@ template <class list_node> class List
 
   void insert (list_node *node)
   {
-    List_insert (head, node);
+    List_insert (mx, head, node);
   }
 
   void remove (list_node *node)