diff options
authorDavid Jeffery <>2019-12-17 11:00:24 -0500
committerJens Axboe <>2019-12-20 16:51:54 -0700
commitdf034c93f15ee71df231ff9fe311d27ff08a2a52 (patch)
parent3b7995a98ad76da5597b488fa84aa5a56d43b608 (diff)
sbitmap: only queue kyber's wait callback if not already activeblock-5.5-20191221
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 <> Signed-off-by: David Jeffery <> Reported-by: John Pittman <> Tested-by: John Pittman <> Signed-off-by: Jens Axboe <>
1 files changed, 1 insertions, 1 deletions
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 33feec8989f1..af88d1346dd7 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -650,8 +650,8 @@ void sbitmap_add_wait_queue(struct sbitmap_queue *sbq,
if (!sbq_wait->sbq) {
sbq_wait->sbq = sbq;
+ add_wait_queue(&ws->wait, &sbq_wait->wait);
- add_wait_queue(&ws->wait, &sbq_wait->wait);