From 3b9185076b63d1e24cd43209a7a9c26e78b659bf Mon Sep 17 00:00:00 2001
From: Joe Thornber <thornber@redhat.com>
Date: Fri, 27 Jun 2014 15:29:04 -0400
Subject: [PATCH] dm io: fix a race condition in the wake up code for sync_io

commit 10f1d5d111e8aed46a0f1179faf9a3cf422f689e upstream.

There's a race condition between the atomic_dec_and_test(&io->count)
in dec_count() and the waking of the sync_io() thread.  If the thread
is spuriously woken immediately after the decrement it may exit,
making the on stack io struct invalid, yet the dec_count could still
be using it.

Fix this race by using a completion in sync_io() and dec_count().

Reported-by: Minfei Huang <huangminfei@ucloud.cn>
Signed-off-by: Joe Thornber <thornber@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Mikulas Patocka <mpatocka@redhat.com>
[bwh: Backported to 3.2: use wait_for_completion() as wait_for_completion_io()
 is not available]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
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