Use atomic ops to access lock_owner in WIN32 tor_mutex_t #17927

2 files changed, 7 insertions, 5 deletions

diff --git a/src/lib/lock/compat_mutex.h b/src/lib/lock/compat_mutex.h
index 14bdf70ce5..518ba96b53 100644
--- a/src/lib/lock/compat_mutex.h
+++ b/src/lib/lock/compat_mutex.h
@@ -46,7 +46,7 @@ typedef struct tor_mutex_t {
     NON_RECURSIVE = 0,
     RECURSIVE
   } type;
-  DWORD lock_owner; // id of the thread that owns the lock
+  LONG lock_owner; // id of the thread that owns the lock
   int lock_count; // number of times the lock is held recursively
 #elif defined(USE_PTHREADS)
   /** Pthreads-only: with pthreads, we implement locks with
diff --git a/src/lib/lock/compat_mutex_winthreads.c b/src/lib/lock/compat_mutex_winthreads.c
index 03cf5082e0..151a7b80f7 100644
--- a/src/lib/lock/compat_mutex_winthreads.c
+++ b/src/lib/lock/compat_mutex_winthreads.c
@@ -58,13 +58,15 @@ tor_mutex_uninit(tor_mutex_t *m)
 static void
 tor_mutex_acquire_recursive(tor_mutex_t *m)
 {
-  DWORD thread_id = GetCurrentThreadId();
-  if (thread_id == m->lock_owner) {
+  LONG thread_id = GetCurrentThreadId();
+  // use InterlockedCompareExchange to perform an atomic read
+  LONG lock_owner = InterlockedCompareExchange(&m->lock_owner, 0, 0);
+  if (thread_id == lock_owner) {
     ++m->lock_count;
     return;
   }
   AcquireSRWLockExclusive(&m->mutex);
-  m->lock_owner = thread_id;
+  InterlockedExchange(&m->lock_owner, thread_id);
   m->lock_count = 1;
 }
 
@@ -91,7 +93,7 @@ tor_mutex_release_recursive(tor_mutex_t *m)
   if (--m->lock_count) {
     return;
   }
-  m->lock_owner = 0;
+  InterlockedExchange(&m->lock_owner, 0);
   ReleaseSRWLockExclusive(&m->mutex);
 }