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: 52ebea749aae ("writeback: make backing_dev_info host cgroup-specific bdi_writebacks")
Co-developed-by: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: Anders Roxell <anders.roxell@xxxxxxxxxx>
---
include/linux/backing-dev-defs.h | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index 9a6bc0951cfa..c31157135598 100644
--- 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);
}
--
2.19.1
