linux-kernel - [PATCH 5.4 130/165] sbitmap: only queue kybers wait callback if not already active

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Date:   Sat, 11 Jan 2020 10:50:49 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Omar Sandoval <osandov@...com>,
        David Jeffery <djeffery@...hat.com>,
        John Pittman <jpittman@...hat.com>,
        Jens Axboe <axboe@...nel.dk>, Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.4 130/165] sbitmap: only queue kybers wait callback if not already active

From: David Jeffery <djeffery@...hat.com>

[ Upstream commit df034c93f15ee71df231ff9fe311d27ff08a2a52 ]

Under heavy loads where the kyber I/O scheduler hits the token limits for
its scheduling domains, kyber can become stuck.  When active requests
complete, kyber may not be woken up leaving the I/O requests in kyber
stuck.

This stuck state is due to a race condition with kyber and the sbitmap
functions it uses to run a callback when enough requests have completed.
The running of a sbt_wait callback can race with the attempt to insert the
sbt_wait.  Since sbitmap_del_wait_queue removes the sbt_wait from the list
first then sets the sbq field to NULL, kyber can see the item as not on a
list but the call to sbitmap_add_wait_queue will see sbq as non-NULL. This
results in the sbt_wait being inserted onto the wait list but ws_active
doesn't get incremented.  So the sbitmap queue does not know there is a
waiter on a wait list.

Since sbitmap doesn't think there is a waiter, kyber may never be
informed that there are domain tokens available and the I/O never advances.
With the sbt_wait on a wait list, kyber believes it has an active waiter
so cannot insert a new waiter when reaching the domain's full state.

This race can be fixed by only adding the sbt_wait to the queue if the
sbq field is NULL.  If sbq is not NULL, there is already an action active
which will trigger the re-running of kyber.  Let it run and add the
sbt_wait to the wait list if still needing to wait.

Reviewed-by: Omar Sandoval <osandov@...com>
Signed-off-by: David Jeffery <djeffery@...hat.com>
Reported-by: John Pittman <jpittman@...hat.com>
Tested-by: John Pittman <jpittman@...hat.com>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 lib/sbitmap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 969e5400a615..ee3ce1494568 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -667,8 +667,8 @@ void sbitmap_add_wait_queue(struct sbitmap_queue *sbq,
 	if (!sbq_wait->sbq) {
 		sbq_wait->sbq = sbq;
 		atomic_inc(&sbq->ws_active);
+		add_wait_queue(&ws->wait, &sbq_wait->wait);
 	}
-	add_wait_queue(&ws->wait, &sbq_wait->wait);
 }
 EXPORT_SYMBOL_GPL(sbitmap_add_wait_queue);

-- 
2.20.1