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 simulates rendezvous with shared memory, pause, and SIGUSR1.
32  */
33 
34 #include <signal.h>
35 #include <lib9.h>
36 
37 enum
38 {
39 	VOUSHASH = 257,
40 };
41 
42 typedef struct Vous Vous;
43 struct Vous
44 {
45 	Vous *link;
46 	Lock lk;
47 	int pid;
48 	ulong val;
49 	ulong tag;
50 };
51 
52 static void
53 ign(int x)
54 {
55 	USED(x);
56 }
57 
58 void /*__attribute__((constructor))*/
59 ignusr1(void)
60 {
61 	signal(SIGUSR1, ign);
62 }
63 
64 static Vous vouspool[2048];
65 static int nvousused;
66 static Vous *vousfree;
67 static Vous *voushash[VOUSHASH];
68 static Lock vouslock;
69 
70 static Vous*
71 getvous(void)
72 {
73 	Vous *v;
74 
75 	if(vousfree){
76 		v = vousfree;
77 		vousfree = v->link;
78 	}else if(nvousused < nelem(vouspool))
79 		v = &vouspool[nvousused++];
80 	else
81 		abort();
82 	return v;
83 }
84 
85 static void
86 putvous(Vous *v)
87 {
88 	lock(&vouslock);
89 	v->link = vousfree;
90 	vousfree = v;
91 	unlock(&vouslock);
92 }
93 
94 static Vous*
95 findvous(ulong tag, ulong val, int pid)
96 {
97 	int h;
98 	Vous *v, **l;
99 
100 	lock(&vouslock);
101 	h = tag%VOUSHASH;
102 	for(l=&voushash[h], v=*l; v; l=&(*l)->link, v=*l){
103 		if(v->tag == tag){
104 			*l = v->link;
105 			unlock(&vouslock);
106 			return v;
107 		}
108 	}
109 	v = getvous();
110 	v->pid = pid;
111 	v->link = voushash[h];
112 	v->val = val;
113 	v->tag = tag;
114 	lock(&v->lk);
115 	voushash[h] = v;
116 	unlock(&vouslock);
117 	return v;
118 }
119 
120 #define DBG 0
121 ulong
122 rendezvous(ulong tag, ulong val)
123 {
124 	int me, vpid;
125 	ulong rval;
126 	Vous *v;
127 	sigset_t mask;
128 
129 	me = getpid();
130 	v = findvous(tag, val, me);
131 	if(v->pid == me){
132 		if(DBG)fprint(2, "pid is %d tag %lux, sleeping\n", me, tag);
133 		/*
134 		 * No rendezvous partner was found; the next guy
135 		 * through will find v and wake us, so we must go
136 		 * to sleep.
137 		 *
138 		 * To go to sleep:
139 		 *	1. disable USR1 signals.
140 		 *	2. unlock v->lk (tells waker okay to signal us).
141 		 *	3. atomically suspend and enable USR1 signals.
142 		 *
143 		 * The call to ignusr1() could be done once at 
144 		 * process creation instead of every time through rendezvous.
145 		 */
146 		v->val = val;
147 		ignusr1();
148 		sigprocmask(SIG_SETMASK, NULL, &mask);
149 		sigaddset(&mask, SIGUSR1);
150 		sigprocmask(SIG_SETMASK, &mask, NULL);
151 		sigdelset(&mask, SIGUSR1);
152 		unlock(&v->lk);
153 		sigsuspend(&mask);
154 		rval = v->val;
155 		if(DBG)fprint(2, "pid is %d, awake\n", me);
156 		putvous(v);
157 	}else{
158 		/*
159 		 * Found someone to meet.  Wake him:
160 		 *
161 		 *	A. lock v->lk (waits for him to get to his step 2)
162 		 *	B. send a USR1
163 		 *
164 		 * He won't get the USR1 until he suspends, which
165 		 * means it must wake him up (it can't get delivered
166 		 * before he sleeps).
167 		 */
168 		vpid = v->pid;
169 		lock(&v->lk);
170 		rval = v->val;
171 		v->val = val;
172 		unlock(&v->lk);
173 		if(kill(vpid, SIGUSR1) < 0){
174 			if(DBG)fprint(2, "pid is %d, kill %d failed: %r\n", me, vpid);
175 			abort();
176 		}
177 	}
178 	return rval;
179 }
180