ThreadError: deadlock; recursive locking

https://sentry.gitlab.net/gitlab/gitlabcom/issues/4098987/?referrer=gitlab_plugin

ThreadError: deadlock; recursive locking
  lib/gitlab/database/load_balancing/host_list.rb:64:in `synchronize'
    @mutex.synchronize do
  lib/gitlab/database/load_balancing/host_list.rb:64:in `next_host'
    @mutex.synchronize do
  lib/gitlab/database/load_balancing/host_list.rb:47:in `next'
    next_host.tap do |_|
  lib/gitlab/database/load_balancing/load_balancer.rb:147:in `host'
    request_cache[CACHE_KEY] ||= @host_list.next
  lib/gitlab/database/load_balancing/load_balancer.rb:50:in `read'
    while host
...
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