1 /*
2      NAME
3           rendezvous - user level process synchronization
4 
5      SYNOPSIS
6           ulong rendezvous(ulong tag, ulong value)
7 
8      DESCRIPTION
9           The rendezvous system call allows two processes to synchro-
10           nize and exchange a value.  In conjunction with the shared
11           memory system calls (see segattach(2) and fork(2)), it
12           enables parallel programs to control their scheduling.
13 
14           Two processes wishing to synchronize call rendezvous with a
15           common tag, typically an address in memory they share.  One
16           process will arrive at the rendezvous first; it suspends
17           execution until a second arrives.  When a second process
18           meets the rendezvous the value arguments are exchanged
19           between the processes and returned as the result of the
20           respective rendezvous system calls.  Both processes are
21           awakened when the rendezvous succeeds.
22 
23           The set of tag values which two processes may use to
24           rendezvous-their tag space-is inherited when a process
25           forks, unless RFREND is set in the argument to rfork; see
26           fork(2).
27 
28           If a rendezvous is interrupted the return value is ~0, so
29           that value should not be used in normal communication.
30 
31  * This assumes we're using pthreads and simulates rendezvous using
32  * shared memory and mutexes.
33  */
34 
35 #include <u.h>
36 #include <pthread.h>
37 #include <signal.h>
38 #include <libc.h>
39 
40 enum
41 {
42 	VOUSHASH = 257,
43 };
44 
45 typedef struct Vous Vous;
46 struct Vous
47 {
48 	Vous *link;
49 	Lock lk;
50 	ulong val;
51 	ulong tag;
52 	pthread_mutex_t mutex;
53 };
54 
55 static void
56 ign(int x)
57 {
58 	USED(x);
59 }
60 
61 void /*__attribute__((constructor))*/
62 ignusr1(void)
63 {
64 	signal(SIGUSR1, ign);
65 }
66 
67 static Vous vouspool[2048];
68 static int nvousused;
69 static Vous *vousfree;
70 static Vous *voushash[VOUSHASH];
71 static Lock vouslock;
72 
73 static Vous*
74 getvous(void)
75 {
76 	Vous *v;
77 
78 	if(vousfree){
79 		v = vousfree;
80 		vousfree = v->link;
81 	}else if(nvousused < nelem(vouspool)){
82 		v = &vouspool[nvousused++];
83 		pthread_mutex_init(&v->mutex, NULL);
84 	}else
85 		abort();
86 	return v;
87 }
88 
89 static void
90 putvous(Vous *v)
91 {
92 	lock(&vouslock);
93 	v->link = vousfree;
94 	vousfree = v;
95 	unlock(&vouslock);
96 }
97 
98 static Vous*
99 findvous(ulong tag, ulong val, int *found)
100 {
101 	int h;
102 	Vous *v, **l;
103 
104 	lock(&vouslock);
105 	h = tag%VOUSHASH;
106 	for(l=&voushash[h], v=*l; v; l=&(*l)->link, v=*l){
107 		if(v->tag == tag){
108 			*l = v->link;
109 			*found = 1;
110 			unlock(&vouslock);
111 			return v;
112 		}
113 	}
114 	v = getvous();
115 	v->link = voushash[h];
116 	v->val = val;
117 	v->tag = tag;
118 	lock(&v->lk);
119 	voushash[h] = v;
120 	unlock(&vouslock);
121 	*found = 0;
122 	return v;
123 }
124 
125 #define DBG 0
126 ulong
127 rendezvous(ulong tag, ulong val)
128 {
129 	int found;
130 	ulong rval;
131 	Vous *v;
132 
133 	v = findvous(tag, val, &found);
134 	if(!found){
135 		if(DBG)fprint(2, "tag %lux, sleeping on %p\n", tag, v);
136 		/*
137 		 * No rendezvous partner was found; the next guy
138 		 * through will find v and wake us, so we must go
139 		 * to sleep.  Do this by locking the mutex (it is
140 		 * unlocked) and then locking it again (our waker will
141 		 * unlock it for us).
142 		 */
143 		if(pthread_mutex_lock(&v->mutex) != 0)
144 			abort();
145 		unlock(&v->lk);
146 		if(pthread_mutex_lock(&v->mutex) != 0)
147 			abort();
148 		rval = v->val;
149 		pthread_mutex_unlock(&v->mutex);
150 		if(DBG)fprint(2, " awake on %p\n", v);
151 		unlock(&v->lk);
152 		putvous(v);
153 	}else{
154 		/*
155 		 * Found someone to meet.  Wake him:
156 		 *
157 		 *	A. lock v->lk (waits for him to lock the mutex once.
158 		 *	B. unlock the mutex (wakes him up)
159 		 */
160 		if(DBG)fprint(2, "found tag %lux on %p, waking\n", tag, v);
161 		lock(&v->lk);
162 		rval = v->val;
163 		v->val = val;
164 		if(pthread_mutex_unlock(&v->mutex) != 0)
165 			abort();
166 		/* lock passes to him */
167 	}
168 	return rval;
169 }
170