src/ipc/ReadWriteLock.h

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