docs: Document the Finisher class.

Signed-off-by: Krzysztof Kosiński <krzysztof.kosinski@intel.com>

diff --git a/src/common/Finisher.cc b/src/common/Finisher.cc
index f3f4107dcf1713f85f99ce93388b9421956f5032..7ebbe05622bcb5c1285fcd627ccb0b68940c56f3 100644 (file)
--- a/src/common/Finisher.cc
+++ b/src/common/Finisher.cc
@@ -20,9 +20,11 @@ void Finisher::stop()
   ldout(cct, 10) << __func__ << dendl;
   finisher_lock.Lock();
   finisher_stop = true;
+  // we don't have any new work to do, but we want the worker to wake up anyway
+  // to process the stop condition.
   finisher_cond.Signal();
   finisher_lock.Unlock();
-  finisher_thread.join();
+  finisher_thread.join(); // wait until the worker exits completely
   ldout(cct, 10) << __func__ << " finish" << dendl;
 }
 
@@ -43,7 +45,10 @@ void *Finisher::finisher_thread_entry()
   ldout(cct, 10) << "finisher_thread start" << dendl;
 
   while (!finisher_stop) {
+    /// Every time we are woken up, we process the queue until it is empty.
     while (!finisher_queue.empty()) {
+      // To reduce lock contention, we swap out the queue to process.
+      // This way other threads can submit new contexts to complete while we are working.
       vector<Context*> ls;
       list<pair<Context*,int> > ls_rval;
       ls.swap(finisher_queue);
@@ -52,12 +57,17 @@ void *Finisher::finisher_thread_entry()
       finisher_lock.Unlock();
       ldout(cct, 10) << "finisher_thread doing " << ls << dendl;
 
+      // Now actually process the contexts.
       for (vector<Context*>::iterator p = ls.begin();
           p != ls.end();
           ++p) {
        if (*p) {
          (*p)->complete(0);
        } else {
+         // When an item is NULL in the finisher_queue, it means
+         // we should instead process an item from finisher_queue_rval,
+         // which has a parameter for complete() other than zero.
+         // This preserves the order while saving some storage.
          assert(!ls_rval.empty());
          Context *c = ls_rval.front().first;
          c->complete(ls_rval.front().second);
@@ -80,6 +90,8 @@ void *Finisher::finisher_thread_entry()
     ldout(cct, 10) << "finisher_thread sleeping" << dendl;
     finisher_cond.Wait(finisher_lock);
   }
+  // If we are exiting, we signal the thread waiting in stop(),
+  // otherwise it would never unblock
   finisher_empty_cond.Signal();
 
   ldout(cct, 10) << "finisher_thread stop" << dendl;

diff --git a/src/common/Finisher.h b/src/common/Finisher.h
index 3a5b4e393efb05fd3192295f140ecfb417f9cce1..876744599031614d317f46ffba643a5b699393ed 100644 (file)
--- a/src/common/Finisher.h
+++ b/src/common/Finisher.h
@@ -23,19 +23,36 @@
 
 class CephContext;
 
+/// Finisher queue length performance counter ID.
 enum {
   l_finisher_first = 997082,
   l_finisher_queue_len,
   l_finisher_last
 };
 
+/** @brief Asynchronous cleanup class.
+ * Finisher asynchronously completes Contexts, which are simple classes
+ * representing callbacks, in a dedicated worker thread. Enqueuing
+ * contexts to complete is thread-safe.
+ */
 class Finisher {
   CephContext *cct;
-  Mutex          finisher_lock;
-  Cond           finisher_cond, finisher_empty_cond;
-  bool           finisher_stop, finisher_running;
+  Mutex        finisher_lock; ///< Protects access to queues and finisher_running.
+  Cond         finisher_cond; ///< Signaled when there is something to process.
+  Cond         finisher_empty_cond; ///< Signaled when the finisher has nothing more to process.
+  bool         finisher_stop; ///< Set when the finisher should stop.
+  bool         finisher_running; ///< True when the finisher is currently executing contexts.
+  /// Queue for contexts for which complete(0) will be called.
+  /// NULLs in this queue indicate that an item from finisher_queue_rval
+  /// should be completed in that place instead.
   vector<Context*> finisher_queue;
+
+  /// Queue for contexts for which the complete function will be called
+  /// with a parameter other than 0.
   list<pair<Context*,int> > finisher_queue_rval;
+
+  /// Performance counter for the finisher's queue length.
+  /// Only active for named finishers.
   PerfCounters *logger;
   
   void *finisher_thread_entry();
@@ -47,6 +64,7 @@ class Finisher {
   } finisher_thread;
 
  public:
+  /// Add a context to complete, optionally specifying a parameter for the complete function.
   void queue(Context *c, int r = 0) {
     finisher_lock.Lock();
     if (finisher_queue.empty()) {
@@ -94,17 +112,32 @@ class Finisher {
     finisher_lock.Unlock();
     ls.clear();
   }
-  
+
+  /// Start the worker thread.
   void start();
+
+  /** @brief Stop the worker thread.
+   *
+   * Does not wait until all outstanding contexts are completed.
+   * To ensure that everything finishes, you should first shut down
+   * all sources that can add contexts to this finisher and call
+   * wait_for_empty() before calling stop(). */
   void stop();
 
+  /** @brief Blocks until the finisher has nothing left to process.
+   * This function will also return when a concurrent call to stop()
+   * finishes, but this class should never be used in this way. */
   void wait_for_empty();
 
+  /// Construct an anonymous Finisher.
+  /// Anonymous finishers do not log their queue length.
   Finisher(CephContext *cct_) :
     cct(cct_), finisher_lock("Finisher::finisher_lock"),
     finisher_stop(false), finisher_running(false),
     logger(0),
     finisher_thread(this) {}
+
+  /// Construct a named Finisher that logs its queue length.
   Finisher(CephContext *cct_, string name) :
     cct(cct_), finisher_lock("Finisher::finisher_lock"),
     finisher_stop(false), finisher_running(false),
@@ -126,6 +159,7 @@ class Finisher {
   }
 };
 
+/// Context that is completed asynchronously on the supplied finisher.
 class C_OnFinisher : public Context {
   Context *con;
   Finisher *fin;