From: Benjamin Coddington bcodding@redhat.com
[ Upstream commit ed0172af5d6fc07d1b40ca82f5ca3979300369f7 ]
We've observed NFS clients with sync tasks sleeping in __rpc_execute waiting on RPC_TASK_QUEUED that have not responded to a wake-up from rpc_make_runnable(). I suspect this problem usually goes unnoticed, because on a busy client the task will eventually be re-awoken by another task completion or xprt event. However, if the state manager is draining the slot table, a sync task missing a wake-up can result in a hung client.
We've been able to prove that the waker in rpc_make_runnable() successfully calls wake_up_bit() (ie- there's no race to tk_runstate), but the wake_up_bit() call fails to wake the waiter. I suspect the waker is missing the load of the bit's wait_queue_head, so waitqueue_active() is false. There are some very helpful comments about this problem above wake_up_bit(), prepare_to_wait(), and waitqueue_active().
Fix this by inserting smp_mb__after_atomic() before the wake_up_bit(), which pairs with prepare_to_wait() calling set_current_state().
Signed-off-by: Benjamin Coddington bcodding@redhat.com Reviewed-by: Jeff Layton jlayton@kernel.org Signed-off-by: Anna Schumaker Anna.Schumaker@Netapp.com Signed-off-by: Sasha Levin sashal@kernel.org --- net/sunrpc/sched.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c index a5c6a3d05741a..df83c59cde7fe 100644 --- a/net/sunrpc/sched.c +++ b/net/sunrpc/sched.c @@ -368,8 +368,10 @@ static void rpc_make_runnable(struct workqueue_struct *wq, if (RPC_IS_ASYNC(task)) { INIT_WORK(&task->u.tk_work, rpc_async_schedule); queue_work(wq, &task->u.tk_work); - } else + } else { + smp_mb__after_atomic(); wake_up_bit(&task->tk_runstate, RPC_TASK_QUEUED); + } }
/*