src/ipc/ReadWriteLock.h

   1 /*
   2  * Copyright (C) 1996-2020 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 <atomic>
  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     ReadWriteLock() : readers(0), writing(false), appending(false), readLevel(0), writeLevel(0)
  29     {}
  30
  31     bool lockShared(); ///< lock for reading or return false
  32     bool lockExclusive(); ///< lock for modification or return false
  33     bool lockHeaders(); ///< lock for [readable] metadata update or return false
  34     void unlockShared(); ///< undo successful sharedLock()
  35     void unlockExclusive(); ///< undo successful exclusiveLock()
  36     void unlockHeaders(); ///< undo successful lockHeaders()
  37     void switchExclusiveToShared(); ///< stop writing, start reading
  38     /// same as unlockShared() but also attempts to get a writer lock beforehand
  39     /// \returns whether the writer lock was acquired
  40     bool unlockSharedAndSwitchToExclusive();
  41
  42     void startAppending(); ///< writer keeps its lock but also allows reading
  43
  44     /// adds approximate current stats to the supplied ones
  45     void updateStats(ReadWriteLockStats &stats) const;
  46
  47 public:
  48     mutable std::atomic<uint32_t> readers; ///< number of reading users
  49     std::atomic<bool> writing; ///< there is a writing user (there can be at most 1)
  50     std::atomic<bool> appending; ///< the writer has promised to only append
  51     std::atomic_flag updating; ///< a reader is updating metadata/headers
  52
  53 private:
  54     mutable std::atomic<uint32_t> readLevel; ///< number of users reading (or trying to)
  55     std::atomic<uint32_t> writeLevel; ///< number of users writing (or trying to write)
  56 };
  57
  58 /// approximate stats of a set of ReadWriteLocks
  59 class ReadWriteLockStats
  60 {
  61 public:
  62     ReadWriteLockStats();
  63
  64     void dump(StoreEntry &e) const;
  65
  66     int count; ///< the total number of locks
  67     int readable; ///< number of locks locked for reading
  68     int writeable; ///< number of locks locked for writing
  69     int idle; ///< number of unlocked locks
  70     int readers; ///< sum of lock.readers
  71     int writers; ///< sum of lock.writers
  72     int appenders; ///< number of appending writers
  73 };
  74
  75 /// Same as assert(flag is set): The process assert()s if flag is not set.
  76 /// Side effect: The unset flag becomes set just before we assert().
  77 /// Needed because atomic_flag cannot be compared with a boolean.
  78 void AssertFlagIsSet(std::atomic_flag &flag);
  79
  80 } // namespace Ipc
  81
  82 #endif /* SQUID_IPC_READ_WRITE_LOCK_H */
  83