[PATCH v2 2/2] writeback: don't decrement wb->refcnt if !wb->bdi

From: Anders Roxell <anders.roxell@linaro.org>
To: linux@armlinux.org.uk, gregkh@linuxfoundation.org,
	akpm@linux-foundation.org
Cc: linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org,
	tj@kernel.org, Anders Roxell <anders.roxell@linaro.org>,
	Arnd Bergmann <arnd@arndb.de>
Subject: [PATCH v2 2/2] writeback: don't decrement wb->refcnt if !wb->bdi
Date: Tue, 30 Oct 2018 12:35:45 +0100	[thread overview]
Message-ID: <20181030113545.30999-2-anders.roxell@linaro.org> (raw)
In-Reply-To: <20181030113545.30999-1-anders.roxell@linaro.org>

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@arndb.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
---
 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

