sbitmap: only queue kyber's wait callback if not already active

BugLink: https://bugs.launchpad.net/bugs/1860130
[ 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@fb.com>
Signed-off-by: David Jeffery <djeffery@redhat.com>
Reported-by: John Pittman <jpittman@redhat.com>
Tested-by: John Pittman <jpittman@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 969e5400a61533f988e81267786e19947e0b7f43..ee3ce1494568c63cabbdd7738837518d77bf5736 100644 (file)
--- 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);