re: Btrfs: fix num_workers_starting bug and other bugs in async thread

[Dan Carpenter <dan.carpenter@oracle.com>]

Hi Josef,

Smatch complains about this change introduces a double unlock.

fs/btrfs/async-thread.c +608 find_worker(49) error: double unlock 'spin_lock:&workers->lock'

   579                          spin_unlock_irqrestore(&workers->lock, flags);
                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
We unlock here.

   580                          /* we're below the limit, start another worker */
   581                          ret = __btrfs_start_workers(workers);
   582                          if (ret)
   583                                  goto fallback;
   584                          goto again;
   585                  }
   586          }
   587          goto found;
   588  
   589  fallback:
   590          fallback = NULL;
   591          /*
   592           * we have failed to find any workers, just
   593           * return the first one we can find.
   594           */
   595          if (!list_empty(&workers->worker_list))
   596                  fallback = workers->worker_list.next;
   597          if (!list_empty(&workers->idle_list))
   598                  fallback = workers->idle_list.next;
   599          BUG_ON(!fallback);
   600          worker = list_entry(fallback,
   601                    struct btrfs_worker_thread, worker_list);
   602  found:
   603          /*
   604           * this makes sure the worker doesn't exit before it is placed
   605           * onto a busy/idle list
   606           */
   607          atomic_inc(&worker->num_pending);
   608          spin_unlock_irqrestore(&workers->lock, flags);
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
And again here.

Btw, does find_worker() ever get called with IRQs disabled?  If so then
__btrfs_start_workers() enables them.  Maybe that function should use
spin_lock_irqsave() instead of spin_lock_irq().

regards,
dan carpenter