diff --git a/winsup/cygwin/shm.cc b/winsup/cygwin/shm.cc
index 88d3c0a48..b60ce4cde 100644
--- a/winsup/cygwin/shm.cc
+++ b/winsup/cygwin/shm.cc
@@ -113,9 +113,9 @@ struct shm_attached_list {
 
 static SLIST_HEAD (, shm_attached_list) sph_list;
 
-static NO_COPY muto shm_guard;
-#define SLIST_LOCK()	(shm_guard.init ("shm_guard")->acquire ())
-#define SLIST_UNLOCK()	(shm_guard.release ())
+static NO_COPY SRWLOCK shm_lock = SRWLOCK_INIT;
+#define SLIST_LOCK()    (AcquireSRWLockExclusive (&shm_lock))
+#define SLIST_UNLOCK()  (ReleaseSRWLockExclusive (&shm_lock))
 
 int
 fixup_shms_after_fork ()
@@ -174,22 +174,22 @@ shmat (int shmid, const void *shmaddr, int shmflg)
 	 shmid if one exists.  Since shmctl inserts a new entry for this
 	 shmid into ssh_list automatically, we just have to go through
 	 that list again.  If that still fails, well, bad luck. */
+      SLIST_UNLOCK ();
       if (shmid && shmget ((key_t) shmid, 0, IPC_KEY_IS_SHMID) != -1)
 	{
+	  SLIST_LOCK ();
 	  SLIST_FOREACH (ssh_entry, &ssh_list, ssh_next)
 	    {
 	      if (ssh_entry->shmid == shmid)
-		break;
+		goto inc_ref_count;
 	    }
-	}
-      if (!ssh_entry)
-	{
-	  /* Invalid shmid */
-	  set_errno (EINVAL);
 	  SLIST_UNLOCK ();
-	  return (void *) -1;
 	}
+      /* Invalid shmid */
+      set_errno (EINVAL);
+      return (void *) -1;
     }
+inc_ref_count:
   /* Early increment ref counter.  This allows further actions to run with
      unlocked lists, because shmdt or shmctl(IPC_RMID) won't delete this
      ssh_entry. */