If the sending queue has a task without ->rq_cong set at the front,
and then a number of tasks with ->rq_cong set such that they use
the entire congestion window, then the queue deadlocks. The first
entry cannot be processed until later entries complete.
This scenario has been seen with a client using UDP to access a server,
and the network connection breaking for a period of time - it doesn't
recover.
It never really makes sense for an ->rq_cong request to be on the ->sending
queue, but it can happen when a request is being retried, and finds
the transport if locked (XPRT_LOCKED). In this case we simple call
__xprt_put_cong() and the deadlock goes away.
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
2980731811