[英]Why does the mutex lock twice in LevelDB?
I'm learning the levelDB C++ project, here is the situation, Status s = Write(WriteOptions(), nullptr)
triggers a compaction work, and then enter the while
loop to wait the signal, the compaction thread goes to BackgroundCall
method, and it also needs to lock the mutex_
, I'm not sure the TEST_CompactMemTable
still holds the mutex, so I print the debug message in the Lock
and Unlock
method.本人在学习levelDB C++项目,情况如下,
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BackgroundCall
方法,它还需要锁定mutex_
,我不确定TEST_CompactMemTable
仍然持有互斥锁,所以我在Lock
and Unlock
方法中打印了调试消息。 The output is just like:输出就像:
TEST_CompactMemTable mutex lock
BackgroundCallmutex lock
BackgroundCallmutex unlock
TEST_CompactMemTable mutex unlock
I'm confused why does the mutex lock twice?我很困惑为什么互斥锁会锁定两次? Am i missing something, any help would be greatly appreciated.
我错过了什么,任何帮助将不胜感激。
Status DBImpl::TEST_CompactMemTable() {
Status s = Write(WriteOptions(), nullptr);
if (s.ok()) {
// lock the mutex first
MutexLock l(&mutex_);
while (imm_ != nullptr && bg_error_.ok()) {
background_work_finished_signal_.Wait();
}
if (imm_ != nullptr) {
s = bg_error_;
}
}
return s;
}
void DBImpl::BackgroundCall() {
// lock the mutex twice
MutexLock l(&mutex_);
assert(background_compaction_scheduled_);
if (shutting_down_.load(std::memory_order_acquire)) {
} else if (!bg_error_.ok()) {
} else {
BackgroundCompaction();
}
background_compaction_scheduled_ = false;
MaybeScheduleCompaction();
background_work_finished_signal_.SignalAll();
}
This is simple background_work_finished_signal_
is conditional variable which is associated with a mutex_
.这是简单的
background_work_finished_signal_
是与mutex_
关联的条件变量。
This is done during construction: see DBImpl::DBImpl这是在构建过程中完成的: 参见 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_)) {}
Now when background_work_finished_signal_.Wait();
现在当
background_work_finished_signal_.Wait();
is called mutex have to be released, so other thread can lock it and send notifications.被称为 mutex 必须被释放,所以其他线程可以锁定它并发送通知。 When notification is received lock is restored before
background_work_finished_signal_.Wait();
当
background_work_finished_signal_.Wait();
之前收到通知时恢复锁background_work_finished_signal_.Wait();
returns control.返回控制。
So basically those logs of yours are from different threads and mutex is unlock and lock by background_work_finished_signal_.Wait();
所以基本上你的那些日志来自不同的线程,互斥锁由
background_work_finished_signal_.Wait();
解锁和锁定background_work_finished_signal_.Wait();
and your logs do not spot it.并且您的日志没有发现它。
So infact your logs should print something like this:所以事实上你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
When the code execution goes to background_work_finished_signal_.Wait()
the mutex gets unlocked and gets locked again after the signal raised.当代码执行到
background_work_finished_signal_.Wait()
,互斥锁被解锁并在信号发出后再次被锁定。 You don't see it, because you don't log messages in the mutex methods.您看不到它,因为您没有在互斥方法中记录消息。 The real flow should be
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
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