Cygwin: shm: Convert muto into SRWLOCK and avoid overlocking

shmat may call shmget.  shmget locks by itself as necessary,
so there's no reason to keep the lock active and recurse into
the lock.  Use SRWLOCK and only lock  as required.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

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. */