我在学习levelDB C++项目,情况是这样的,Status s = Write(WriteOptions(), nullptr)
触发compaction工作,然后进入while
循环等待信号,compaction线程去{{1 }} 方法,它也需要锁定 BackgroundCall
,我不确定 mutex_
是否仍然持有互斥锁,所以我在 TEST_CompactMemTable
和 {{1 }} 方法。
输出就像:
Lock
我很困惑为什么互斥锁会锁定两次?我是否遗漏了什么,任何帮助将不胜感激。
Unlock
TEST_CompactMemTable mutex lock
BackgroundCallmutex lock
BackgroundCallmutex unlock
TEST_CompactMemTable mutex unlock
答案 0 :(得分:1)
这很简单,background_work_finished_signal_
是与 mutex_
关联的条件变量。
这是在构建过程中完成的:see DBImpl::DBImpl
DBImpl::DBImpl(const Options& raw_options, const std::string& dbname)
: env_(raw_options.env),
internal_comparator_(raw_options.comparator),
internal_filter_policy_(raw_options.filter_policy),
options_(SanitizeOptions(dbname, &internal_comparator_,
&internal_filter_policy_, raw_options)),
owns_info_log_(options_.info_log != raw_options.info_log),
owns_cache_(options_.block_cache != raw_options.block_cache),
dbname_(dbname),
table_cache_(new TableCache(dbname_, options_, TableCacheSize(options_))),
db_lock_(nullptr),
shutting_down_(false),
background_work_finished_signal_(&mutex_),
mem_(nullptr),
imm_(nullptr),
has_imm_(false),
logfile_(nullptr),
logfile_number_(0),
log_(nullptr),
seed_(0),
tmp_batch_(new WriteBatch),
background_compaction_scheduled_(false),
manual_compaction_(nullptr),
versions_(new VersionSet(dbname_, &options_, table_cache_,
&internal_comparator_)) {}
现在当 background_work_finished_signal_.Wait();
被调用时必须释放互斥锁,以便其他线程可以锁定它并发送通知。
当收到通知时,在 background_work_finished_signal_.Wait();
返回控制之前恢复锁定。
所以基本上你的那些日志来自不同的线程,互斥锁被 background_work_finished_signal_.Wait();
解锁和锁定,你的日志没有发现它。
事实上,您的日志应该打印如下内容:
TEST_CompactMemTable mutex lock
TEST_CompactMemTable_wait_cv mutex unlock
BackgroundCallmutex lock
BackgroundCallmutex unlock
TEST_CompactMemTable_wait_cv mutex lock
TEST_CompactMemTable mutex unlock
答案 1 :(得分:1)
当代码执行到 background_work_finished_signal_.Wait()
时,互斥锁被解锁并在信号发出后再次被锁定。您看不到它,因为您没有在互斥方法中记录消息。真正的流量应该是
TEST_CompactMemTable mutex lock
signal wait
TEST_CompactMemTable mutex unlock
BackgroundCallmutex mutex lock
signal raised
BackgroundCallmutex mutex unlock
TEST_CompactMemTable mutex lock
TEST_CompactMemTable mutex unlock