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  24 
  25 #ifndef SHARE_RUNTIME_BIASEDLOCKING_HPP
  26 #define SHARE_RUNTIME_BIASEDLOCKING_HPP
  27 
  28 #include "runtime/handles.hpp"
  29 #include "utilities/growableArray.hpp"
  30 
  31 // This class describes operations to implement Store-Free Biased
  32 // Locking. The high-level properties of the scheme are similar to
  33 // IBM's lock reservation, Dice-Moir-Scherer QR locks, and other biased
  34 // locking mechanisms. The principal difference is in the handling of
  35 // recursive locking which is how this technique achieves a more
  36 // efficient fast path than these other schemes.
  37 //
  38 // The basic observation is that in HotSpot's current fast locking
  39 // scheme, recursive locking (in the fast path) causes no update to
  40 // the object header. The recursion is described simply by stack
  41 // records containing a specific value (NULL). Only the last unlock by
  42 // a given thread causes an update to the object header.
  43 //
  44 // This observation, coupled with the fact that HotSpot only compiles
  45 // methods for which monitor matching is obeyed (and which therefore
  46 // can not throw IllegalMonitorStateException), implies that we can
  47 // completely eliminate modifications to the object header for
  48 // recursive locking in compiled code, and perform similar recursion
  49 // checks and throwing of IllegalMonitorStateException in the
  50 // interpreter with little or no impact on the performance of the fast
  51 // path.
  52 //
  53 // The basic algorithm is as follows (note, see below for more details
  54 // and information). A pattern in the low three bits is reserved in
  55 // the object header to indicate whether biasing of a given object's
  56 // lock is currently being done or is allowed at all.  If the bias
  57 // pattern is present, the contents of the rest of the header are
  58 // either the JavaThread* of the thread to which the lock is biased,
  59 // or NULL, indicating that the lock is "anonymously biased". The
  60 // first thread which locks an anonymously biased object biases the
  61 // lock toward that thread. If another thread subsequently attempts to
  62 // lock the same object, the bias is revoked.
  63 //
  64 // Because there are no updates to the object header at all during
  65 // recursive locking while the lock is biased, the biased lock entry
  66 // code is simply a test of the object header's value. If this test
  67 // succeeds, the lock has been acquired by the thread. If this test
  68 // fails, a bit test is done to see whether the bias bit is still
  69 // set. If not, we fall back to HotSpot's original CAS-based locking
  70 // scheme. If it is set, we attempt to CAS in a bias toward this
  71 // thread. The latter operation is expected to be the rarest operation
  72 // performed on these locks. We optimistically expect the biased lock
  73 // entry to hit most of the time, and want the CAS-based fallthrough
  74 // to occur quickly in the situations where the bias has been revoked.
  75 //
  76 // Revocation of the lock's bias is fairly straightforward. We want to
  77 // restore the object's header and stack-based BasicObjectLocks and
  78 // BasicLocks to the state they would have been in had the object been
  79 // locked by HotSpot's usual fast locking scheme. To do this, we bring
  80 // the system to a safepoint and walk the stack of the thread toward
  81 // which the lock is biased. We find all of the lock records on the
  82 // stack corresponding to this object, in particular the first /
  83 // "highest" record. We fill in the highest lock record with the
  84 // object's displaced header (which is a well-known value given that
  85 // we don't maintain an identity hash nor age bits for the object
  86 // while it's in the biased state) and all other lock records with 0,
  87 // the value for recursive locks. When the safepoint is released, the
  88 // formerly-biased thread and all other threads revert back to
  89 // HotSpot's CAS-based locking.
  90 //
  91 // This scheme can not handle transfers of biases of single objects
  92 // from thread to thread efficiently, but it can handle bulk transfers
  93 // of such biases, which is a usage pattern showing up in some
  94 // applications and benchmarks. We implement "bulk rebias" and "bulk
  95 // revoke" operations using a "bias epoch" on a per-data-type basis.
  96 // If too many bias revocations are occurring for a particular data
  97 // type, the bias epoch for the data type is incremented at a
  98 // safepoint, effectively meaning that all previous biases are
  99 // invalid. The fast path locking case checks for an invalid epoch in
 100 // the object header and attempts to rebias the object with a CAS if
 101 // found, avoiding safepoints or bulk heap sweeps (the latter which
 102 // was used in a prior version of this algorithm and did not scale
 103 // well). If too many bias revocations persist, biasing is completely
 104 // disabled for the data type by resetting the prototype header to the
 105 // unbiased markOop. The fast-path locking code checks to see whether
 106 // the instance's bias pattern differs from the prototype header's and
 107 // causes the bias to be revoked without reaching a safepoint or,
 108 // again, a bulk heap sweep.
 109 
 110 // Biased locking counters
 111 class BiasedLockingCounters {
 112  private:
 113   int _total_entry_count;
 114   int _biased_lock_entry_count;
 115   int _anonymously_biased_lock_entry_count;
 116   int _rebiased_lock_entry_count;
 117   int _revoked_lock_entry_count;
 118   int _fast_path_entry_count;
 119   int _slow_path_entry_count;
 120 
 121  public:
 122   BiasedLockingCounters() :
 123     _total_entry_count(0),
 124     _biased_lock_entry_count(0),
 125     _anonymously_biased_lock_entry_count(0),
 126     _rebiased_lock_entry_count(0),
 127     _revoked_lock_entry_count(0),
 128     _fast_path_entry_count(0),
 129     _slow_path_entry_count(0) {}
 130 
 131   int slow_path_entry_count(); // Compute this field if necessary
 132 
 133   int* total_entry_count_addr()                   { return &_total_entry_count; }
 134   int* biased_lock_entry_count_addr()             { return &_biased_lock_entry_count; }
 135   int* anonymously_biased_lock_entry_count_addr() { return &_anonymously_biased_lock_entry_count; }
 136   int* rebiased_lock_entry_count_addr()           { return &_rebiased_lock_entry_count; }
 137   int* revoked_lock_entry_count_addr()            { return &_revoked_lock_entry_count; }
 138   int* fast_path_entry_count_addr()               { return &_fast_path_entry_count; }
 139   int* slow_path_entry_count_addr()               { return &_slow_path_entry_count; }
 140 
 141   bool nonzero() { return _total_entry_count > 0; }
 142 
 143   void print_on(outputStream* st);
 144   void print() { print_on(tty); }
 145 };
 146 
 147 
 148 class BiasedLocking : AllStatic {
 149 private:
 150   static BiasedLockingCounters _counters;
 151 
 152 public:
 153   static int* total_entry_count_addr();
 154   static int* biased_lock_entry_count_addr();
 155   static int* anonymously_biased_lock_entry_count_addr();
 156   static int* rebiased_lock_entry_count_addr();
 157   static int* revoked_lock_entry_count_addr();
 158   static int* fast_path_entry_count_addr();
 159   static int* slow_path_entry_count_addr();
 160 
 161   enum Condition {
 162     NOT_BIASED = 1,
 163     BIAS_REVOKED = 2,
 164     BIAS_REVOKED_AND_REBIASED = 3
 165   };
 166 
 167   // This initialization routine should only be called once and
 168   // schedules a PeriodicTask to turn on biased locking a few seconds
 169   // into the VM run to avoid startup time regressions
 170   static void init();
 171 
 172   // This provides a global switch for leaving biased locking disabled
 173   // for the first part of a run and enabling it later
 174   static bool enabled();
 175 
 176   // This should be called by JavaThreads to revoke the bias of an object
 177   static Condition revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS);
 178 
 179   // These do not allow rebiasing; they are used by deoptimization to
 180   // ensure that monitors on the stack can be migrated
 181   static void revoke(GrowableArray<Handle>* objs);
 182   static void revoke_at_safepoint(Handle obj);
 183   static void revoke_at_safepoint(GrowableArray<Handle>* objs);
 184 
 185   static void print_counters() { _counters.print(); }
 186   static BiasedLockingCounters* counters() { return &_counters; }
 187 
 188   // These routines are GC-related and should not be called by end
 189   // users. GCs which do not do preservation of mark words do not need
 190   // to call these routines.
 191   static void preserve_marks();
 192   static void restore_marks();
 193 };
 194 
 195 #endif // SHARE_RUNTIME_BIASEDLOCKING_HPP