From: Anders Roxell <anders.roxell@xxxxxxxxxx> Subject: writeback: don't decrement wb->refcnt if !wb->bdi 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. Link: http://lkml.kernel.org/r/20181030113545.30999-2-anders.roxell@xxxxxxxxxx Fixes: 52ebea749aae ("writeback: make backing_dev_info host cgroup-specific bdi_writebacks") Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx> Signed-off-by: Anders Roxell <anders.roxell@xxxxxxxxxx> Co-developed-by: Arnd Bergmann <arnd@xxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- --- a/include/linux/backing-dev-defs.h~writeback-dont-decrement-wb-refcnt-if-wb-bdi +++ a/include/linux/backing-dev-defs.h @@ -258,6 +258,14 @@ static inline void wb_get(struct bdi_wri */ 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); } _
