src/ipc/ReadWriteLock.h

   1 /*
   2  * Copyright (C) 1996-2015 The Squid Software Foundation and contributors
   3  *
   4  * Squid software is distributed under GPLv2+ license and includes
   5  * contributions from numerous individuals and organizations.
   6  * Please see the COPYING and CONTRIBUTORS files for details.
   7  */
   8
   9 #ifndef SQUID_IPC_READ_WRITE_LOCK_H
  10 #define SQUID_IPC_READ_WRITE_LOCK_H
  11
  12 #include "ipc/AtomicWord.h"
  13
  14 class StoreEntry;
  15
  16 namespace Ipc
  17 {
  18
  19 class ReadWriteLockStats;
  20
  21 /// an atomic readers-writer or shared-exclusive lock suitable for maps/tables
  22 /// Also supports reading-while-appending mode when readers and writer are
  23 /// allowed to access the same locked object because the writer promisses
  24 /// to only append new data and all size-related object properties are atomic.
  25 class ReadWriteLock
  26 {
  27 public:
  28     // default constructor is OK because of shared memory zero-initialization
  29
  30     bool lockShared(); ///< lock for reading or return false
  31     bool lockExclusive(); ///< lock for modification or return false
  32     void unlockShared(); ///< undo successful sharedLock()
  33     void unlockExclusive(); ///< undo successful exclusiveLock()
  34     void switchExclusiveToShared(); ///< stop writing, start reading
  35
  36     void startAppending(); ///< writer keeps its lock but also allows reading
  37
  38     /// adds approximate current stats to the supplied ones
  39     void updateStats(ReadWriteLockStats &stats) const;
  40
  41 public:
  42     mutable Atomic::Word readers; ///< number of reading users
  43     Atomic::Word writing; ///< there is a writing user (there can be at most 1)
  44     Atomic::Word appending; ///< the writer has promissed to only append
  45
  46 private:
  47     mutable Atomic::Word readLevel; ///< number of users reading (or trying to)
  48     Atomic::Word writeLevel; ///< number of users writing (or trying to write)
  49 };
  50
  51 /// approximate stats of a set of ReadWriteLocks
  52 class ReadWriteLockStats
  53 {
  54 public:
  55     ReadWriteLockStats();
  56
  57     void dump(StoreEntry &e) const;
  58
  59     int count; ///< the total number of locks
  60     int readable; ///< number of locks locked for reading
  61     int writeable; ///< number of locks locked for writing
  62     int idle; ///< number of unlocked locks
  63     int readers; ///< sum of lock.readers
  64     int writers; ///< sum of lock.writers
  65     int appenders; ///< number of appending writers
  66 };
  67
  68 } // namespace Ipc
  69
  70 #endif /* SQUID_IPC_READ_WRITE_LOCK_H */
  71