Add helper of blk_mq_global_quiesce_wait() for supporting to quiesce
queues in parallel, then we can just wait once if global quiesce wait
is allowed.
blk_mq_global_quiesce_wait() is a poor name... global is scope-less and
obviously it has a scope.
How about blk_mq_shared_quiesce_wait()? or any suggestion?
Shared between what?
Maybe if the queue has a non-blocking tagset, it can have a "quiesced"
flag that is cleared in unquiesce? then the callers can just continue
to iterate but will only wait the rcu grace period once.
