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 <u.h>
35 typedef u32int u32;
36 #include <errno.h>
37 #include <sys/time.h>
38 #define __user
39 #include <linux/linkage.h>
40 #include <linux/futex.h>
41 #include <libc.h>
42 
43 enum
44 {
45 	VOUSHASH = 257,
46 };
47 
48 typedef struct Vous Vous;
49 struct Vous
50 {
51 	Vous *link;
52 	Lock lk;
53 	int pid;
54 	ulong val;
55 	ulong tag;
56 };
57 
58 static Vous vouspool[2048];
59 static int nvousused;
60 static Vous *vousfree;
61 static Vous *voushash[VOUSHASH];
62 static Lock vouslock;
63 
64 static Vous*
65 getvous(void)
66 {
67 	Vous *v;
68 
69 	if(vousfree){
70 		v = vousfree;
71 		vousfree = v->link;
72 	}else if(nvousused < nelem(vouspool))
73 		v = &vouspool[nvousused++];
74 	else
75 		abort();
76 	return v;
77 }
78 
79 static void
80 putvous(Vous *v)
81 {
82 	lock(&vouslock);
83 	v->link = vousfree;
84 	vousfree = v;
85 	unlock(&vouslock);
86 }
87 
88 static Vous*
89 findvous(ulong tag, ulong val, int pid)
90 {
91 	int h;
92 	Vous *v, **l;
93 
94 	lock(&vouslock);
95 	h = tag%VOUSHASH;
96 	for(l=&voushash[h], v=*l; v; l=&(*l)->link, v=*l){
97 		if(v->tag == tag){
98 			*l = v->link;
99 			unlock(&vouslock);
100 			return v;
101 		}
102 	}
103 	v = getvous();
104 	v->pid = pid;
105 	v->link = voushash[h];
106 	v->val = val;
107 	v->tag = tag;
108 	lock(&v->lk);
109 	voushash[h] = v;
110 	unlock(&vouslock);
111 	return v;
112 }
113 
114 #define DBG 0
115 ulong
116 rendezvous(ulong tag, ulong val)
117 {
118 	int me, vpid;
119 	ulong rval;
120 	Vous *v;
121 
122 	me = getpid();
123 	v = findvous(tag, val, me);
124 	if(v->pid == me){
125 		if(DBG)fprint(2, "pid is %d tag %lux, sleeping\n", me, tag);
126 		/*
127 		 * No rendezvous partner was found; the next guy
128 		 * through will find v and wake us, so we must go
129 		 * to sleep.
130 		 *
131 		 * To go to sleep:
132 		 *	1. disable USR1 signals.
133 		 *	2. unlock v->lk (tells waker okay to signal us).
134 		 *	3. atomically suspend and enable USR1 signals.
135 		 *
136 		 * The call to ignusr1() could be done once at 
137 		 * process creation instead of every time through rendezvous.
138 		 */
139 		v->val = val;
140 		unlock(&v->lk);
141 		while(sys_futex((u32int*)&v->tag, FUTEX_WAIT, tag, nil, nil) < 0 && errno==EINTR)
142 			;
143 		rval = v->val;
144 		if(DBG)fprint(2, "pid is %d, awake\n", me);
145 		putvous(v);
146 	}else{
147 		/*
148 		 * Found someone to meet.  Wake him:
149 		 *
150 		 *	A. lock v->lk (waits for him to get to his step 2)
151 		 *	B. send a USR1
152 		 *
153 		 * He won't get the USR1 until he suspends, which
154 		 * means it must wake him up (it can't get delivered
155 		 * before he sleeps).
156 		 */
157 		vpid = v->pid;
158 		lock(&v->lk);
159 		rval = v->val;
160 		v->val = val;
161 		v->tag++;
162 		unlock(&v->lk);
163 		sys_futex((u32int*)&v->tag, FUTEX_WAKE, 1, nil, nil);
164 	}
165 	return rval;
166 }
167