writeback: don't decrement wb->refcnt if !wb->bdi

[ Upstream commit 347a28b ] This happened while running in qemu-system-aarch64, the AMBA PL011 UART driver when enabling CONFIG_DEBUG_TEST_DRIVER_REMOVE. arch_initcall(pl011_init) came before subsys_initcall(default_bdi_init), devtmpfs' handle_remove() crashes because the reference count is a NULL pointer only because wb->bdi hasn't been initialized yet. Rework so that wb_put have an extra check if wb->bdi before decrement wb->refcnt and also add a WARN_ON_ONCE to get a warning if it happens again in other drivers. Fixes: 52ebea7 ("writeback: make backing_dev_info host cgroup-specific bdi_writebacks") Co-developed-by: Arnd Bergmann <[email protected]> Signed-off-by: Arnd Bergmann <[email protected]> Signed-off-by: Anders Roxell <[email protected]> Signed-off-by: Greg Kroah-Hartman <[email protected]> Signed-off-by: Sasha Levin <[email protected]>
crow-misia · Jan 26, 2019 · e7a5f00 · e7a5f00
1 parent b6d7542
commit e7a5f00
Showing 1 changed file with 8 additions and 0 deletions.
diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
@@ -258,6 +258,14 @@ static inline void wb_get(struct bdi_writeback *wb)
  */
 static inline void wb_put(struct bdi_writeback *wb)
 {
+	if (WARN_ON_ONCE(!wb->bdi)) {
+		/*
+		 * A driver bug might cause a file to be removed before bdi was
+		 * initialized.
+		 */
+		return;
+	}
+
 	if (wb != &wb->bdi->wb)
 		percpu_ref_put(&wb->refcnt);
 }