From: David Jeffery djeffery@redhat.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@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 --- 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);