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 <pthread.h>
36 #include <signal.h>
37 #include <lib9.h>
38 
39 enum
40 {
41 	VOUSHASH = 257,
42 };
43 
44 typedef struct Vous Vous;
45 struct Vous
46 {
47 	Vous *link;
48 	Lock lk;
49 	ulong val;
50 	ulong tag;
51 	pthread_mutex_t mutex;
52 };
53 
54 static void
55 ign(int x)
56 {
57 	USED(x);
58 }
59 
60 void /*__attribute__((constructor))*/
61 ignusr1(void)
62 {
63 	signal(SIGUSR1, ign);
64 }
65 
66 static Vous vouspool[2048];
67 static int nvousused;
68 static Vous *vousfree;
69 static Vous *voushash[VOUSHASH];
70 static Lock vouslock;
71 
72 static Vous*
73 getvous(void)
74 {
75 	Vous *v;
76 
77 	if(vousfree){
78 		v = vousfree;
79 		vousfree = v->link;
80 	}else if(nvousused < nelem(vouspool)){
81 		v = &vouspool[nvousused++];
82 		pthread_mutex_init(&v->mutex, NULL);
83 	}else
84 		abort();
85 	return v;
86 }
87 
88 static void
89 putvous(Vous *v)
90 {
91 	lock(&vouslock);
92 	v->link = vousfree;
93 	vousfree = v;
94 	unlock(&vouslock);
95 }
96 
97 static Vous*
98 findvous(ulong tag, ulong val, int *found)
99 {
100 	int h;
101 	Vous *v, **l;
102 
103 	lock(&vouslock);
104 	h = tag%VOUSHASH;
105 	for(l=&voushash[h], v=*l; v; l=&(*l)->link, v=*l){
106 		if(v->tag == tag){
107 			*l = v->link;
108 			*found = 1;
109 			unlock(&vouslock);
110 			return v;
111 		}
112 	}
113 	v = getvous();
114 	v->link = voushash[h];
115 	v->val = val;
116 	v->tag = tag;
117 	lock(&v->lk);
118 	voushash[h] = v;
119 	unlock(&vouslock);
120 	*found = 0;
121 	return v;
122 }
123 
124 #define DBG 0
125 ulong
126 rendezvous(ulong tag, ulong val)
127 {
128 	int found;
129 	ulong rval;
130 	Vous *v;
131 
132 	v = findvous(tag, val, &found);
133 	if(!found){
134 		if(DBG)fprint(2, "tag %lux, sleeping on %p\n", tag, v);
135 		/*
136 		 * No rendezvous partner was found; the next guy
137 		 * through will find v and wake us, so we must go
138 		 * to sleep.  Do this by locking the mutex (it is
139 		 * unlocked) and then locking it again (our waker will
140 		 * unlock it for us).
141 		 */
142 		if(pthread_mutex_lock(&v->mutex) != 0)
143 			abort();
144 		unlock(&v->lk);
145 		if(pthread_mutex_lock(&v->mutex) != 0)
146 			abort();
147 		rval = v->val;
148 		pthread_mutex_unlock(&v->mutex);
149 		if(DBG)fprint(2, " awake on %p\n", v);
150 		unlock(&v->lk);
151 		putvous(v);
152 	}else{
153 		/*
154 		 * Found someone to meet.  Wake him:
155 		 *
156 		 *	A. lock v->lk (waits for him to lock the mutex once.
157 		 *	B. unlock the mutex (wakes him up)
158 		 */
159 		if(DBG)fprint(2, "found tag %lux on %p, waking\n", tag, v);
160 		lock(&v->lk);
161 		rval = v->val;
162 		v->val = val;
163 		if(pthread_mutex_unlock(&v->mutex) != 0)
164 			abort();
165 		/* lock passes to him */
166 	}
167 	return rval;
168 }
169