From 2673b4cf5d59c3ee5e0c12f6d734d38770324dc4 Mon Sep 17 00:00:00 2001
From: Rabin Vincent <rabin@rab.in>
Date: Sun, 29 Jan 2012 12:17:33 -0600
Subject: [PATCH] backing-dev: fix wakeup timer races with bdi_unregister()

While 7a401a972df8e18 ("backing-dev: ensure wakeup_timer is deleted")
addressed the problem of the bdi being freed with a queued wakeup
timer, there are other races that could happen if the wakeup timer
expires after/during bdi_unregister(), before bdi_destroy() is called.

wakeup_timer_fn() could attempt to wakeup a task which has already has
been freed, or could access a NULL bdi->dev via the wake_forker_thread
tracepoint.

Cc: <stable@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Reported-by: Chanho Min <chanho.min@lge.com>
Reviewed-by: Namjae Jeon <linkinjeon@gmail.com>
Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
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