jenkins master升级后无法启动slave

惊心动魄一刻，master升级到最新war包，重启动后，10多台slave无法启动。
一下在大数据部的分布式计算都无法运行。
紧急恢复到过去版本的war包，仍然不行，下载了最新版本的war包，重启后依然不行。
最后google，在 https://groups.google.com/forum/#!topic/jenkinsci-users/HgWWqwaR07w里找到2013年出现过一次bug。
Hi, Just upgraded Jenkins to pickup some bug fixes (disappearing job log).
Rekey process is running in the background. Is it not done yet and causing this?

Every single slave failed to launch with this error:

[10/28/13 16:47:39] [SSH] Opening SSH connection to batvm06.perf.lab.tripwire.com:22.
ERROR: Unexpected error in launching a slave. This is probably a bug in Jenkins.
java.lang.NullPointerException
at hudson.plugins.sshslaves.SSHLauncher.pemKeyEquals(SSHLauncher.java:471)
at hudson.plugins.sshslaves.SSHLauncher.access$000(SSHLauncher.java:120)
at hudson.plugins.sshslaves.SSHLauncher$1.matches(SSHLauncher.java:449)
at com.cloudbees.plugins.credentials.matchers.AllOfMatcher.matches(AllOfMatcher.java:63)
at com.cloudbees.plugins.credentials.CredentialsMatchers.firstOrDefault(CredentialsMatchers.java:356)
at com.cloudbees.plugins.credentials.CredentialsMatchers.firstOrNull(CredentialsMatchers.java:374)
at hudson.plugins.sshslaves.SSHLauncher.retrieveExistingCredentials(SSHLauncher.java:436)
at hudson.plugins.sshslaves.SSHLauncher.upgrade(SSHLauncher.java:404)
at hudson.plugins.sshslaves.SSHLauncher.getCredentials(SSHLauncher.java:389)
at hudson.plugins.sshslaves.SSHLauncher.openConnection(SSHLauncher.java:1044)
at hudson.plugins.sshslaves.SSHLauncher.launch(SSHLauncher.java:588)
at hudson.slaves.SlaveComputer$1.call(SlaveComputer.java:230)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
[10/28/13 16:47:39] [SSH] Connection closed.
结论是重新到slave的配置页面，手动保存一下证书然后保存即可。