Make mm writelock testing less intrusive.

This enables it only for debug kernels, and also makes sure
that if some external module is still broken, we don't leave
the mmap-sem locked after warning about it.

mm/mmap.c

@@ -1870,6 +1870,16 @@ asmlinkage long sys_munmap(unsigned long addr, size_t len)
 	return ret;
 }
 
+static inline void verify_mm_writelocked(struct mm_struct *mm)
+{
+#ifdef CONFIG_DEBUG_KERNEL
+	if (unlikely(down_read_trylock(&mm->mmap_sem))) {
+		WARN_ON(1);
+		up_read(&mm->mmap_sem);
+	}
+#endif
+}
+
 /*
  *  this is really a simplified "do_mmap".  it only handles
  *  anonymous maps.  eventually we may be able to do some
@@ -1906,7 +1916,7 @@ unsigned long do_brk(unsigned long addr, unsigned long len)
 	 * mm->mmap_sem is required to protect against another thread
 	 * changing the mappings in case we sleep.
 	 */
-	WARN_ON(down_read_trylock(&mm->mmap_sem));
+	verify_mm_writelocked(mm);
 
 	/*
 	 * Clear old maps.  this also does some error checking for us