diff --git a/LOG b/LOG
index 39a263f9b6..b3b1dfaa5b 100644
--- a/LOG
+++ b/LOG
@@ -248,3 +248,8 @@
     io.ms
 - fixed typo: VC/bin/vcvars64.bat => VC/bin/amd64/vcvars64.bat
     c/Mf-a6nt, c/Mf-ta6nt
+- scheme_mutex_t now uses volatile keyword for owner and count fields
+  because these fields can be accessed from multiple threads
+  concurrently. Updated $yield and $thread-check in mats/thread.ms to
+  be more tolerant of timing variability.
+    c/types.h, mats/thread.ms, release_notes.stex
diff --git a/c/types.h b/c/types.h
index 698acff3de..8273478a34 100644
--- a/c/types.h
+++ b/c/types.h
@@ -277,8 +277,8 @@ typedef struct _bucket_pointer_list {
 
 #ifdef PTHREADS
 typedef struct {
-  s_thread_t owner;
-  uptr count;
+  volatile s_thread_t owner;
+  volatile uptr count;
   s_thread_mutex_t pmutex;
 } scheme_mutex_t;
 
diff --git a/mats/thread.ms b/mats/thread.ms
index 991dff4386..ecf5a98b7f 100644
--- a/mats/thread.ms
+++ b/mats/thread.ms
@@ -94,11 +94,11 @@
     (define $fib (lambda (x) (if (< x 2) 1 (+ ($fib (- x 1)) ($fib (- x 2))))))
     (define $nthreads 1)
     (define $yield
-      (let ([t (make-time 'time-duration 1000 0)])
+      (let ([t (make-time 'time-duration 1000000 0)])
         (lambda () (sleep t))))
     (define $thread-check
       (lambda ()
-        (let loop ([n 10] [nt (length ($threads))])
+        (let loop ([n 100] [nt (length ($threads))])
           (cond
             [(<= nt $nthreads)
              (set! $nthreads nt)
diff --git a/release_notes/release_notes.stex b/release_notes/release_notes.stex
index db3d811953..43f153e859 100644
--- a/release_notes/release_notes.stex
+++ b/release_notes/release_notes.stex
@@ -1474,6 +1474,12 @@ On Windows, the inliner has been fixed to handle \scheme{wchar} and
 
 On Windows, the path-related procedures now raise an appropriate exception when the path argument is not a string.
 
+\subsection{Internal mutex structure not always thread safe (9.4)}
+
+The internal mutex structure now has its fields marked
+\scheme{volatile} so that the C compiler does not reorder assignments
+to these fields.
+
 \subsection{\protect\scheme{dynamic-wind} mistakenly enabling interrupts (9.3.3)}
 
 A bug causing \scheme{dynamic-wind} to unconditionally enable