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#ifndef SHARE_VM_RUNTIME_MUTEX_HPP
#define SHARE_VM_RUNTIME_MUTEX_HPP
#include "memory/allocation.hpp"
#include "runtime/os.hpp"
#include "utilities/histogram.hpp"
// The SplitWord construct allows us to colocate the contention queue
// (cxq) with the lock-byte. The queue elements are ParkEvents, which are
// always aligned on 256-byte addresses - the least significant byte of
// a ParkEvent is always 0. Colocating the lock-byte with the queue
// allows us to easily avoid what would otherwise be a race in lock()
// if we were to use two completely separate fields for the contention queue
// and the lock indicator. Specifically, colocation renders us immune
// from the race where a thread might enqueue itself in the lock() slow-path
// immediately after the lock holder drops the outer lock in the unlock()
// fast-path.
//
// Colocation allows us to use a fast-path unlock() form that uses
// A MEMBAR instead of a CAS. MEMBAR has lower local latency than CAS
// on many platforms.
//
// See:
// + http://blogs.sun.com/dave/entry/biased_locking_in_hotspot
// + http://blogs.sun.com/dave/resource/synchronization-public2.pdf
//
// Note that we're *not* using word-tearing the classic sense.
// The lock() fast-path will CAS the lockword and the unlock()
// fast-path will store into the lock-byte colocated within the lockword.
// We depend on the fact that all our reference platforms have
// coherent and atomic byte accesses. More precisely, byte stores
// interoperate in a safe, sane, and expected manner with respect to
// CAS, ST and LDs to the full-word containing the byte.
// If you're porting HotSpot to a platform where that isn't the case
// then you'll want change the unlock() fast path from:
// STB;MEMBAR #storeload; LDN
// to a full-word CAS of the lockword.
union SplitWord { // full-word with separately addressable LSB
volatile intptr_t FullWord ;
volatile void * Address ;
volatile jbyte Bytes [sizeof(intptr_t)] ;
} ;
class ParkEvent ;
// See orderAccess.hpp. We assume throughout the VM that mutex lock and
// try_lock do fence-lock-acquire, and that unlock does a release-unlock,
// *in that order*. If their implementations change such that these
// assumptions are violated, a whole lot of code will break.
// The default length of monitor name was originally chosen to be 64 to avoid
// false sharing. Now, PaddedMonitor is available for this purpose.
// TODO: Check if _name[MONITOR_NAME_LEN] should better get replaced by const char*.
static const int MONITOR_NAME_LEN = 64;
class Monitor : public CHeapObj<mtInternal> {
public:
// A special lock: Is a lock where you are guaranteed not to block while you are
// holding it, i.e., no vm operation can happen, taking other (blocking) locks, etc.
// The rank 'access' is similar to 'special' and has the same restrictions on usage.
// It is reserved for locks that may be required in order to perform memory accesses
// that require special barriers, e.g. SATB GC barriers, that in turn uses locks.
// Since memory accesses should be able to be performed pretty much anywhere
// in the code, that requires locks required for performing accesses being
// inherently a bit more special than even locks of the 'special' rank.
// NOTE: It is critical that the rank 'special' be the lowest (earliest)
// (except for "event" and "access") for the deadlock detection to work correctly.
// The rank native is only for use in Mutex's created by JVM_RawMonitorCreate,
// which being external to the VM are not subject to deadlock detection.
// The rank safepoint is used only for synchronization in reaching a
// safepoint and leaving a safepoint. It is only used for the Safepoint_lock
// currently. While at a safepoint no mutexes of rank safepoint are held
// by any thread.
// The rank named "leaf" is probably historical (and should
// be changed) -- mutexes of this rank aren't really leaf mutexes
// at all.
enum lock_types {
event,
access = event + 1,
special = access + 2,
suspend_resume = special + 1,
leaf = suspend_resume + 2,
safepoint = leaf + 10,
barrier = safepoint + 1,
nonleaf = barrier + 1,
max_nonleaf = nonleaf + 900,
native = max_nonleaf + 1
};
// The WaitSet and EntryList linked lists are composed of ParkEvents.
// I use ParkEvent instead of threads as ParkEvents are immortal and
// type-stable, meaning we can safely unpark() a possibly stale
// list element in the unlock()-path.
protected: // Monitor-Mutex metadata
SplitWord _LockWord ; // Contention queue (cxq) colocated with Lock-byte
Thread * volatile _owner; // The owner of the lock
// Consider sequestering _owner on its own $line
// to aid future synchronization mechanisms.
ParkEvent * volatile _EntryList ; // List of threads waiting for entry
ParkEvent * volatile _OnDeck ; // heir-presumptive
volatile intptr_t _WaitLock [1] ; // Protects _WaitSet
ParkEvent * volatile _WaitSet ; // LL of ParkEvents
volatile bool _snuck; // Used for sneaky locking (evil).
int NotifyCount ; // diagnostic assist
char _name[MONITOR_NAME_LEN]; // Name of mutex
// Debugging fields for naming, deadlock detection, etc. (some only used in debug mode)
#ifndef PRODUCT
bool _allow_vm_block;
debug_only(int _rank;) // rank (to avoid/detect potential deadlocks)
debug_only(Monitor * _next;) // Used by a Thread to link up owned locks
debug_only(Thread* _last_owner;) // the last thread to own the lock
debug_only(static bool contains(Monitor * locks, Monitor * lock);)
debug_only(static Monitor * get_least_ranked_lock(Monitor * locks);)
debug_only(Monitor * get_least_ranked_lock_besides_this(Monitor * locks);)
#endif
void set_owner_implementation(Thread* owner) PRODUCT_RETURN;
void check_prelock_state (Thread* thread) PRODUCT_RETURN;
void check_block_state (Thread* thread) PRODUCT_RETURN;
// platform-dependent support code can go here (in os_<os_family>.cpp)
public:
enum {
_no_safepoint_check_flag = true,
_allow_vm_block_flag = true,
_as_suspend_equivalent_flag = true
};
// Locks can be acquired with or without safepoint check.
// Monitor::lock and Monitor::lock_without_safepoint_check
// checks these flags when acquiring a lock to ensure
// consistent checking for each lock.
// A few existing locks will sometimes have a safepoint check and
// sometimes not, but these locks are set up in such a way to avoid deadlocks.
enum SafepointCheckRequired {
_safepoint_check_never, // Monitors with this value will cause errors
// when acquired with a safepoint check.
_safepoint_check_sometimes, // Certain locks are called sometimes with and
// sometimes without safepoint checks. These
// locks will not produce errors when locked.
_safepoint_check_always // Causes error if locked without a safepoint
// check.
};
NOT_PRODUCT(SafepointCheckRequired _safepoint_check_required;)
enum WaitResults {
CONDVAR_EVENT, // Wait returned because of condition variable notification
INTERRUPT_EVENT, // Wait returned because waiting thread was interrupted
NUMBER_WAIT_RESULTS
};
private:
int TrySpin (Thread * Self) ;
int TryLock () ;
int TryFast () ;
int AcquireOrPush (ParkEvent * ev) ;
void IUnlock (bool RelaxAssert) ;
void ILock (Thread * Self) ;
int IWait (Thread * Self, jlong timo);
int ILocked () ;
protected:
static void ClearMonitor (Monitor * m, const char* name = NULL) ;
Monitor() ;
public:
Monitor(int rank, const char *name, bool allow_vm_block = false,
SafepointCheckRequired safepoint_check_required = _safepoint_check_always);
~Monitor();
// Wait until monitor is notified (or times out).
// Defaults are to make safepoint checks, wait time is forever (i.e.,
// zero), and not a suspend-equivalent condition. Returns true if wait
// times out; otherwise returns false.
bool wait(bool no_safepoint_check = !_no_safepoint_check_flag,
long timeout = 0,
bool as_suspend_equivalent = !_as_suspend_equivalent_flag);
bool notify();
bool notify_all();
void lock(); // prints out warning if VM thread blocks
void lock(Thread *thread); // overloaded with current thread
void unlock();
bool is_locked() const { return _owner != NULL; }
bool try_lock(); // Like lock(), but unblocking. It returns false instead
// Lock without safepoint check. Should ONLY be used by safepoint code and other code
// that is guaranteed not to block while running inside the VM.
void lock_without_safepoint_check();
void lock_without_safepoint_check (Thread * Self) ;
// Current owner - not not MT-safe. Can only be used to guarantee that
// the current running thread owns the lock
Thread* owner() const { return _owner; }
bool owned_by_self() const;
// Support for JVM_RawMonitorEnter & JVM_RawMonitorExit. These can be called by
// non-Java thread. (We should really have a RawMonitor abstraction)
void jvm_raw_lock();
void jvm_raw_unlock();
const char *name() const { return _name; }
void print_on_error(outputStream* st) const;
#ifndef PRODUCT
void print_on(outputStream* st) const;
void print() const { print_on(tty); }
debug_only(int rank() const { return _rank; })
bool allow_vm_block() { return _allow_vm_block; }
debug_only(Monitor *next() const { return _next; })
debug_only(void set_next(Monitor *next) { _next = next; })
#endif
void set_owner(Thread* owner) {
#ifndef PRODUCT
set_owner_implementation(owner);
debug_only(void verify_Monitor(Thread* thr));
#else
_owner = owner;
#endif
}
};
class PaddedMonitor : public Monitor {
enum {
CACHE_LINE_PADDING = (int)DEFAULT_CACHE_LINE_SIZE - (int)sizeof(Monitor),
PADDING_LEN = CACHE_LINE_PADDING > 0 ? CACHE_LINE_PADDING : 1
};
char _padding[PADDING_LEN];
public:
PaddedMonitor(int rank, const char *name, bool allow_vm_block = false,
SafepointCheckRequired safepoint_check_required = _safepoint_check_always) :
Monitor(rank, name, allow_vm_block, safepoint_check_required) {};
};
// Normally we'd expect Monitor to extend Mutex in the sense that a monitor
// constructed from pthreads primitives might extend a mutex by adding
// a condvar and some extra metadata. In fact this was the case until J2SE7.
//
// Currently, however, the base object is a monitor. Monitor contains all the
// logic for wait(), notify(), etc. Mutex extends monitor and restricts the
// visibility of wait(), notify(), and notify_all().
//
// Another viable alternative would have been to have Monitor extend Mutex and
// implement all the normal mutex and wait()-notify() logic in Mutex base class.
// The wait()-notify() facility would be exposed via special protected member functions
// (e.g., _Wait() and _Notify()) in Mutex. Monitor would extend Mutex and expose wait()
// as a call to _Wait(). That is, the public wait() would be a wrapper for the protected
// _Wait().
//
// An even better alternative is to simply eliminate Mutex:: and use Monitor:: instead.
// After all, monitors are sufficient for Java-level synchronization. At one point in time
// there may have been some benefit to having distinct mutexes and monitors, but that time
// has past.
//
// The Mutex/Monitor design parallels that of Java-monitors, being based on
// thread-specific park-unpark platform-specific primitives.
class Mutex : public Monitor { // degenerate Monitor
public:
Mutex(int rank, const char *name, bool allow_vm_block = false,
SafepointCheckRequired safepoint_check_required = _safepoint_check_always);
// default destructor
private:
bool notify () { ShouldNotReachHere(); return false; }
bool notify_all() { ShouldNotReachHere(); return false; }
bool wait (bool no_safepoint_check, long timeout, bool as_suspend_equivalent) {
ShouldNotReachHere() ;
return false ;
}
};
class PaddedMutex : public Mutex {
enum {
CACHE_LINE_PADDING = (int)DEFAULT_CACHE_LINE_SIZE - (int)sizeof(Mutex),
PADDING_LEN = CACHE_LINE_PADDING > 0 ? CACHE_LINE_PADDING : 1
};
char _padding[PADDING_LEN];
public:
PaddedMutex(int rank, const char *name, bool allow_vm_block = false,
SafepointCheckRequired safepoint_check_required = _safepoint_check_always) :
Mutex(rank, name, allow_vm_block, safepoint_check_required) {};
};
#endif // SHARE_VM_RUNTIME_MUTEX_HPP