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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Oleg Nesterov <oleg@redhat.com>,
        Josef Bacik <josef@toxicpanda.com>,
        Jens Axboe <axboe@kernel.dk>, Sasha Levin <sashal@kernel.org>
Subject: [PATCH 5.2 062/144] rq-qos: use a mb for got_token
Date:   Wed, 14 Aug 2019 19:00:18 +0200
Message-Id: <20190814165802.430179590@linuxfoundation.org>
In-Reply-To: <20190814165759.466811854@linuxfoundation.org>
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[ Upstream commit ac38297f7038cd5b80d66f8809c7bbf5b70031f3 ]

Oleg noticed that our checking of data.got_token is unsafe in the
cleanup case, and should really use a memory barrier.  Use a wmb on the
write side, and a rmb() on the read side.  We don't need one in the main
loop since we're saved by set_current_state().

Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 block/blk-rq-qos.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/block/blk-rq-qos.c b/block/blk-rq-qos.c
index e3ab75e4df9ea..06d024204f504 100644
--- a/block/blk-rq-qos.c
+++ b/block/blk-rq-qos.c
@@ -202,6 +202,7 @@ static int rq_qos_wake_function(struct wait_queue_entry *curr,
 		return -1;
 
 	data->got_token = true;
+	smp_wmb();
 	list_del_init(&curr->entry);
 	wake_up_process(data->task);
 	return 1;
@@ -245,6 +246,7 @@ void rq_qos_wait(struct rq_wait *rqw, void *private_data,
 
 	prepare_to_wait_exclusive(&rqw->wait, &data.wq, TASK_UNINTERRUPTIBLE);
 	do {
+		/* The memory barrier in set_task_state saves us here. */
 		if (data.got_token)
 			break;
 		if (!has_sleeper && acquire_inflight_cb(rqw, private_data)) {
@@ -255,6 +257,7 @@ void rq_qos_wait(struct rq_wait *rqw, void *private_data,
 			 * which means we now have two. Put our local token
 			 * and wake anyone else potentially waiting for one.
 			 */
+			smp_rmb();
 			if (data.got_token)
 				cleanup_cb(rqw, private_data);
 			break;
-- 
2.20.1