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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 execute
  80 // a handshake with the JavaThread that biased the lock. Inside the
  81 // handshake we walk the biaser stack searching for all of the lock
  82 // records corresponding to this object, in particular the first / "highest"
  83 // record. We fill in the highest lock record with the object's displaced
  84 // header (which is a well-known value given that we don't maintain an
  85 // identity hash nor age bits for the object while it's in the biased
  86 // state) and all other lock records with 0, the value for recursive locks.
  87 // Alternatively, we can revoke the bias of an object inside a safepoint
  88 // if we are already in one and we detect that we need to perform a
  89 // revocation.
  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 markWord. 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 _handshakes_count;
 119   int _fast_path_entry_count;
 120   int _slow_path_entry_count;
 121 
 122  public:
 123   BiasedLockingCounters() :
 124     _total_entry_count(0),
 125     _biased_lock_entry_count(0),
 126     _anonymously_biased_lock_entry_count(0),
 127     _rebiased_lock_entry_count(0),
 128     _revoked_lock_entry_count(0),
 129     _handshakes_count(0),
 130     _fast_path_entry_count(0),
 131     _slow_path_entry_count(0) {}
 132 
 133   int slow_path_entry_count() const; // Compute this field if necessary
 134 
 135   int* total_entry_count_addr()                   { return &_total_entry_count; }
 136   int* biased_lock_entry_count_addr()             { return &_biased_lock_entry_count; }
 137   int* anonymously_biased_lock_entry_count_addr() { return &_anonymously_biased_lock_entry_count; }
 138   int* rebiased_lock_entry_count_addr()           { return &_rebiased_lock_entry_count; }
 139   int* revoked_lock_entry_count_addr()            { return &_revoked_lock_entry_count; }
 140   int* handshakes_count_addr()                    { return &_handshakes_count; }
 141   int* fast_path_entry_count_addr()               { return &_fast_path_entry_count; }
 142   int* slow_path_entry_count_addr()               { return &_slow_path_entry_count; }
 143 
 144   bool nonzero() { return _total_entry_count > 0; }
 145 
 146   void print_on(outputStream* st) const;
 147   void print() const;
 148 };
 149 
 150 
 151 class BiasedLocking : AllStatic {
 152 friend class VM_BulkRevokeBias;
 153 friend class RevokeOneBias;
 154 
 155 private:
 156   static BiasedLockingCounters _counters;
 157 
 158 public:
 159   static int* total_entry_count_addr();
 160   static int* biased_lock_entry_count_addr();
 161   static int* anonymously_biased_lock_entry_count_addr();
 162   static int* rebiased_lock_entry_count_addr();
 163   static int* revoked_lock_entry_count_addr();
 164   static int* handshakes_count_addr();
 165   static int* fast_path_entry_count_addr();
 166   static int* slow_path_entry_count_addr();
 167 
 168   enum Condition {
 169     NOT_BIASED = 1,
 170     BIAS_REVOKED = 2,
 171     NOT_REVOKED = 3
 172   };
 173 
 174 private:
 175   static void single_revoke_at_safepoint(oop obj, bool is_bulk, JavaThread* requester, JavaThread** biaser);
 176   static void bulk_revoke_at_safepoint(oop o, bool bulk_rebias, JavaThread* requester);
 177   static Condition single_revoke_with_handshake(Handle obj, JavaThread *requester, JavaThread *biaser);
 178   static void walk_stack_and_revoke(oop obj, JavaThread* biased_locker);
 179 
 180 public:
 181   // This initialization routine should only be called once and
 182   // schedules a PeriodicTask to turn on biased locking a few seconds
 183   // into the VM run to avoid startup time regressions
 184   static void init();
 185 
 186   // This provides a global switch for leaving biased locking disabled
 187   // for the first part of a run and enabling it later
 188   static bool enabled();
 189 
 190   // This should be called by JavaThreads to revoke the bias of an object
 191   static void revoke(Handle obj, TRAPS);
 192 
 193   // This should be called by a JavaThread to revoke the bias of an owned object
 194   static void revoke_own_locks(Handle obj, TRAPS);
 195 
 196   static void revoke_at_safepoint(Handle obj);
 197 
 198   // These are used by deoptimization to ensure that monitors on the stack
 199   // can be migrated
 200   static void revoke(GrowableArray<Handle>* objs, JavaThread *biaser);
 201 
 202   static void print_counters() { _counters.print(); }
 203   static BiasedLockingCounters* counters() { return &_counters; }
 204 
 205   // These routines are GC-related and should not be called by end
 206   // users. GCs which do not do preservation of mark words do not need
 207   // to call these routines.
 208   static void preserve_marks();
 209   static void restore_marks();
 210 };
 211 
 212 #endif // SHARE_RUNTIME_BIASEDLOCKING_HPP