Java的LockSupport.park()实现分析（转载）

 1     void Parker::park(bool isAbsolute, jlong time) {  
 2       // Ideally we‘d do something useful while spinning, such  
 3       // as calling unpackTime().  
 4       
 5       
 6       // Optional fast-path check:  
 7       // Return immediately if a permit is available.  
 8       // We depend on Atomic::xchg() having full barrier semantics  
 9       // since we are doing a lock-free update to _counter.  
10       if (Atomic::xchg(0, &_counter) > 0) return;  

1     ThreadBlockInVM tbivm(jt);  
2     if (_counter > 0)  { // no wait needed  
3       _counter = 0;  
4       status = pthread_mutex_unlock(_mutex);  

1     // We use placement-new to force ParkEvent instances to be  
2     // aligned on 256-byte address boundaries.  This ensures that the least  
3     // significant byte of a ParkEvent address is always 0.  
4        
5     void * operator new (size_t sz) ;  

 1     volatile int Parker::ListLock = 0 ;  
 2     Parker * volatile Parker::FreeList = NULL ;  
 3       
 4     Parker * Parker::Allocate (JavaThread * t) {  
 5       guarantee (t != NULL, "invariant") ;  
 6       Parker * p ;  
 7       
 8       // Start by trying to recycle an existing but unassociated  
 9       // Parker from the global free list.  
10       for (;;) {  
11         p = FreeList ;  
12         if (p  == NULL) break ;  
13         // 1: Detach  
14         // Tantamount to p = Swap (&FreeList, NULL)  
15         if (Atomic::cmpxchg_ptr (NULL, &FreeList, p) != p) {  
16            continue ;  
17         }  
18       
19         // We‘ve detached the list.  The list in-hand is now  
20         // local to this thread.   This thread can operate on the  
21         // list without risk of interference from other threads.  
22         // 2: Extract -- pop the 1st element from the list.  
23         Parker * List = p->FreeNext ;  
24         if (List == NULL) break ;  
25         for (;;) {  
26             // 3: Try to reattach the residual list  
27             guarantee (List != NULL, "invariant") ;  
28             Parker * Arv =  (Parker *) Atomic::cmpxchg_ptr (List, &FreeList, NULL) ;  
29             if (Arv == NULL) break ;  
30       
31             // New nodes arrived.  Try to detach the recent arrivals.  
32             if (Atomic::cmpxchg_ptr (NULL, &FreeList, Arv) != Arv) {  
33                 continue ;  
34             }  
35             guarantee (Arv != NULL, "invariant") ;  
36             // 4: Merge Arv into List  
37             Parker * Tail = List ;  
38             while (Tail->FreeNext != NULL) Tail = Tail->FreeNext ;  
39             Tail->FreeNext = Arv ;  
40         }  
41         break ;  
42       }  
43       
44       if (p != NULL) {  
45         guarantee (p->AssociatedWith == NULL, "invariant") ;  
46       } else {  
47         // Do this the hard way -- materialize a new Parker..  
48         // In rare cases an allocating thread might detach  
49         // a long list -- installing null into FreeList --and  
50         // then stall.  Another thread calling Allocate() would see  
51         // FreeList == null and then invoke the ctor.  In this case we  
52         // end up with more Parkers in circulation than we need, but  
53         // the race is rare and the outcome is benign.  
54         // Ideally, the # of extant Parkers is equal to the  
55         // maximum # of threads that existed at any one time.  
56         // Because of the race mentioned above, segments of the  
57         // freelist can be transiently inaccessible.  At worst  
58         // we may end up with the # of Parkers in circulation  
59         // slightly above the ideal.  
60         p = new Parker() ;  
61       }  
62       p->AssociatedWith = t ;          // Associate p with t  
63       p->FreeNext       = NULL ;  
64       return p ;  
65     }  
66       
67       
68     void Parker::Release (Parker * p) {  
69       if (p == NULL) return ;  
70       guarantee (p->AssociatedWith != NULL, "invariant") ;  
71       guarantee (p->FreeNext == NULL      , "invariant") ;  
72       p->AssociatedWith = NULL ;  
73       for (;;) {  
74         // Push p onto FreeList  
75         Parker * List = FreeList ;  
76         p->FreeNext = List ;  
77         if (Atomic::cmpxchg_ptr (p, &FreeList, List) == List) break ;  
78       }  
79     }