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

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Date:   Tue, 30 Oct 2018 12:35:45 +0100
From:   Anders Roxell <anders.roxell@...aro.org>
To:     linux@...linux.org.uk, gregkh@...uxfoundation.org,
        akpm@...ux-foundation.org
Cc:     linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
        tj@...nel.org, Anders Roxell <anders.roxell@...aro.org>,
        Arnd Bergmann <arnd@...db.de>
Subject: [PATCH v2 2/2] 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.

Fixes: 52ebea749aae ("writeback: make backing_dev_info host cgroup-specific bdi_writebacks")
Co-developed-by: Arnd Bergmann <arnd@...db.de>
Signed-off-by: Arnd Bergmann <arnd@...db.de>
Signed-off-by: Anders Roxell <anders.roxell@...aro.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