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, sigsuspend, and SIGUSR1.
32  */
33 
34 #include <u.h>
35 #include <signal.h>
36 #include <libc.h>
37 
38 #define DBG 0
39 
40 enum
41 {
42 	VOUSHASH = 257,
43 };
44 
45 typedef struct Vous Vous;
46 struct Vous
47 {
48 	Vous *link;
49 	int pid;
50 	int wokeup;
51 	ulong tag;
52 	ulong val1;		/* value for the sleeper */
53 	ulong val2;		/* value for the waker */
54 };
55 
56 static void
57 ign(int x)
58 {
59 	USED(x);
60 }
61 
62 void /*__attribute__((constructor))*/
63 ignusr1(int restart)
64 {
65 	struct sigaction sa;
66 
67 	memset(&sa, 0, sizeof sa);
68 	sa.sa_handler = ign;
69 	sigemptyset(&sa.sa_mask);
70 	sigaddset(&sa.sa_mask, SIGUSR1);
71 	if(restart)
72 		sa.sa_flags = SA_RESTART;
73 	sigaction(SIGUSR1, &sa, nil);
74 }
75 
76 static Vous vouspool[2048];
77 static int nvousused;
78 static Vous *vousfree;
79 static Vous *voushash[VOUSHASH];
80 static Lock vouslock;
81 
82 static Vous*
83 getvous(void)
84 {
85 	Vous *v;
86 
87 	if(vousfree){
88 		v = vousfree;
89 		vousfree = v->link;
90 	}else if(nvousused < nelem(vouspool))
91 		v = &vouspool[nvousused++];
92 	else{
93 		fprint(2, "rendezvous: out of vous!\n");
94 		abort();
95 	}
96 	return v;
97 }
98 
99 static void
100 putvous(Vous *v)
101 {
102 	v->link = vousfree;
103 	vousfree = v;
104 }
105 
106 static Vous*
107 findvous(ulong tag)
108 {
109 	int h;
110 	Vous *v, **l;
111 
112 	h = tag%VOUSHASH;
113 	for(l=&voushash[h], v=*l; v; l=&(*l)->link, v=*l){
114 		if(v->tag == tag){
115 			*l = v->link;
116 			v->link = nil;
117 			return v;
118 		}
119 	}
120 	return nil;
121 }
122 
123 static Vous*
124 mkvous(ulong tag)
125 {
126 	Vous *v;
127 	int h;
128 
129 	h = tag%VOUSHASH;
130 	v = getvous();
131 	v->link = voushash[h];
132 	v->tag = tag;
133 	voushash[h] = v;
134 	return v;
135 }
136 
137 ulong
138 rendezvous(ulong tag, ulong val)
139 {
140 	int vpid, pid;
141 	ulong rval;
142 	Vous *v;
143 	sigset_t mask;
144 
145 	pid = getpid();
146 	lock(&vouslock);
147 	if((v = findvous(tag)) == nil){
148 		/*
149 		 * Go to sleep.
150 		 *
151 		 * Block USR1, set the handler to interrupt system calls,
152 		 * unlock the vouslock so our waker can wake us,
153 		 * and then suspend.
154 		 */
155 		v = mkvous(tag);
156 		v->pid = pid;
157 		v->val2 = val;
158 		v->wokeup = 0;
159 		sigprocmask(SIG_SETMASK, nil, &mask);
160 		sigaddset(&mask, SIGUSR1);
161 		sigprocmask(SIG_SETMASK, &mask, nil);
162 		ignusr1(0);
163 		if(DBG) fprint(2, "%d rv(%lux, %lux) -> s\n", pid, tag, val);
164 		unlock(&vouslock);
165 		sigdelset(&mask, SIGUSR1);
166 		sigsuspend(&mask);
167 
168 		/*
169 		 * We're awake.  Make USR1 not interrupt system calls.
170 		 * Were we awakened or interrupted?
171 		 */
172 		ignusr1(1);
173 		lock(&vouslock);
174 		if(v->wokeup){
175 			rval = v->val1;
176 			if(DBG) fprint(2, "%d rv(%lux, %lux) -> g %lux\n", pid, tag, val, rval);
177 		}else{
178 			if(findvous(tag) != v){
179 				fprint(2, "rendezvous: interrupted but not found in hash table\n");
180 				abort();
181 			}
182 			rval = ~(ulong)0;
183 			if(DBG) fprint(2, "%d rv(%lux, %lux) -> g i\n", pid, tag, val);
184 		}
185 		putvous(v);
186 		unlock(&vouslock);
187 	}else{
188 		/*
189 		 * Wake up sleeper.
190 		 */
191 		rval = v->val2;
192 		v->val1 = val;
193 		vpid = v->pid;
194 		v->wokeup = 1;
195 		if(DBG) fprint(2, "%d rv(%lux, %lux) -> g %lux, w %d\n", pid, tag, val, rval, vpid);
196 		unlock(&vouslock);
197 		kill(vpid, SIGUSR1);
198 	}
199 	return rval;
200 }