交付svn时报告attempted to lock an already-locked dir

提交svn时报告attempted to lock an already-locked dir用subeclipse插件提交代码到svn时报告attempted to

提交svn时报告attempted to lock an already-locked dir
用subeclipse插件提交代码到svn时报告attempted to lock an already-locked dir。

解决方法:
到工程所在目录->.svn,找到lock文件并删除。

这个lock文件时怎么产生的?