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

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[ Upstream commit 347a28b586 ]

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: 52ebea749a ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Anders Roxell
2018-10-30 12:35:45 +01:00
committed by Greg Kroah-Hartman
parent 1894718804
commit b7bf54a9cd
1 changed files with 8 additions and 0 deletions
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8
include/linux/backing-dev-defs.h
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@@ -233,6 +233,14 @@ 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);
}