ref: 4d1d8c342fc391cc8cf4b966a3c4f1d0c756d128
dir: /sys/src/9/port/sysproc.c/
#include "u.h"
#include "tos.h"
#include "../port/lib.h"
#include "mem.h"
#include "dat.h"
#include "fns.h"
#include "../port/error.h"
#include "edf.h"
#include <a.out.h>
uintptr
sysr1(va_list)
{
if(!iseve())
error(Eperm);
return 0;
}
static void
abortion(void)
{
pexit("fork aborted", 1);
}
uintptr
sysrfork(va_list list)
{
Proc *p;
int n, i;
Fgrp *ofg;
Pgrp *opg;
Rgrp *org;
Egrp *oeg;
ulong pid, flag;
Mach *wm;
char *devs;
flag = va_arg(list, ulong);
/* Check flags before we commit */
if((flag & (RFFDG|RFCFDG)) == (RFFDG|RFCFDG))
error(Ebadarg);
if((flag & (RFNAMEG|RFCNAMEG)) == (RFNAMEG|RFCNAMEG))
error(Ebadarg);
if((flag & (RFENVG|RFCENVG)) == (RFENVG|RFCENVG))
error(Ebadarg);
/*
* Code using RFNOMNT expects to block all but
* the following devices.
*/
devs = "|decp";
if((flag&RFPROC) == 0) {
if(flag & (RFMEM|RFNOWAIT))
error(Ebadarg);
if(flag & (RFFDG|RFCFDG)) {
ofg = up->fgrp;
if(flag & RFFDG)
up->fgrp = dupfgrp(ofg);
else
up->fgrp = dupfgrp(nil);
closefgrp(ofg);
}
if(flag & (RFNAMEG|RFCNAMEG)) {
opg = up->pgrp;
up->pgrp = newpgrp();
if(flag & RFNAMEG)
pgrpcpy(up->pgrp, opg);
/* inherit notallowed */
memmove(up->pgrp->notallowed, opg->notallowed, sizeof up->pgrp->notallowed);
closepgrp(opg);
}
if(flag & RFNOMNT)
devmask(up->pgrp, 1, devs);
if(flag & RFREND) {
org = up->rgrp;
up->rgrp = newrgrp();
closergrp(org);
}
if(flag & (RFENVG|RFCENVG)) {
oeg = up->egrp;
up->egrp = smalloc(sizeof(Egrp));
up->egrp->ref = 1;
if(flag & RFENVG)
envcpy(up->egrp, oeg);
closeegrp(oeg);
}
if(flag & RFNOTEG){
qlock(&up->debug);
setnoteid(up, 0); /* can't error() with 0 argument */
qunlock(&up->debug);
}
return 0;
}
if((p = newproc()) == nil)
error("no procs");
qlock(&up->debug);
qlock(&p->debug);
p->scallnr = up->scallnr;
p->s = up->s;
p->slash = up->slash;
p->dot = up->dot;
incref(p->dot);
p->nnote = 0;
p->notify = up->notify;
p->notified = 0;
p->notepending = 0;
p->lastnote = nil;
if((flag & RFNOTEG) == 0)
p->noteid = up->noteid;
p->procmode = up->procmode;
p->privatemem = up->privatemem;
p->noswap = up->noswap;
p->hang = up->hang;
if(up->procctl == Proc_tracesyscall)
p->procctl = Proc_tracesyscall;
p->kp = 0;
/*
* Craft a return frame which will cause the child to pop out of
* the scheduler in user mode with the return register zero
*/
forkchild(p, up->dbgreg);
kstrdup(&p->text, up->text);
kstrdup(&p->user, up->user);
kstrdup(&p->args, "");
p->nargs = 0;
p->setargs = 0;
p->insyscall = 0;
memset(p->time, 0, sizeof(p->time));
p->time[TReal] = MACHP(0)->ticks;
p->kentry = up->kentry;
p->pcycles = -p->kentry;
pid = pidalloc(p);
qunlock(&p->debug);
qunlock(&up->debug);
/* Abort the child process on error */
if(waserror()){
p->kp = 1;
kprocchild(p, abortion);
ready(p);
nexterror();
}
/* Make a new set of memory segments */
n = flag & RFMEM;
qlock(&p->seglock);
if(waserror()){
qunlock(&p->seglock);
nexterror();
}
for(i = 0; i < NSEG; i++)
if(up->seg[i] != nil)
p->seg[i] = dupseg(up->seg, i, n);
qunlock(&p->seglock);
poperror();
/* File descriptors */
if(flag & (RFFDG|RFCFDG)) {
if(flag & RFFDG)
p->fgrp = dupfgrp(up->fgrp);
else
p->fgrp = dupfgrp(nil);
}
else {
p->fgrp = up->fgrp;
incref(p->fgrp);
}
/* Process groups */
if(flag & (RFNAMEG|RFCNAMEG)) {
p->pgrp = newpgrp();
if(flag & RFNAMEG)
pgrpcpy(p->pgrp, up->pgrp);
/* inherit notallowed */
memmove(p->pgrp->notallowed, up->pgrp->notallowed, sizeof p->pgrp->notallowed);
}
else {
p->pgrp = up->pgrp;
incref(p->pgrp);
}
if(flag & RFNOMNT)
devmask(p->pgrp, 1, devs);
if(flag & RFREND)
p->rgrp = newrgrp();
else {
incref(up->rgrp);
p->rgrp = up->rgrp;
}
/* Environment group */
if(flag & (RFENVG|RFCENVG)) {
p->egrp = smalloc(sizeof(Egrp));
p->egrp->ref = 1;
if(flag & RFENVG)
envcpy(p->egrp, up->egrp);
}
else {
p->egrp = up->egrp;
incref(p->egrp);
}
procfork(p);
poperror(); /* abortion */
if((flag&RFNOWAIT) == 0){
p->parent = up;
lock(&up->exl);
up->nchild++;
unlock(&up->exl);
}
/*
* since the bss/data segments are now shareable,
* any mmu info about this process is now stale
* (i.e. has bad properties) and has to be discarded.
*/
flushmmu();
p->basepri = up->basepri;
p->priority = up->basepri;
p->fixedpri = up->fixedpri;
p->mp = up->mp;
wm = up->wired;
if(wm != nil)
procwired(p, wm->machno);
p->psstate = nil;
ready(p);
sched();
return pid;
}
static int
shargs(char *s, int n, char **ap, int nap)
{
char *p;
int i;
if(n <= 2 || s[0] != '#' || s[1] != '!')
return -1;
s += 2;
n -= 2; /* skip #! */
if((p = memchr(s, '\n', n)) == nil)
return 0;
*p = 0;
i = tokenize(s, ap, nap-1);
ap[i] = nil;
return i;
}
ulong
beswal(ulong l)
{
uchar *p;
p = (uchar*)&l;
return (p[0]<<24) | (p[1]<<16) | (p[2]<<8) | p[3];
}
uvlong
beswav(uvlong v)
{
uchar *p;
p = (uchar*)&v;
return ((uvlong)p[0]<<56) | ((uvlong)p[1]<<48) | ((uvlong)p[2]<<40)
| ((uvlong)p[3]<<32) | ((uvlong)p[4]<<24)
| ((uvlong)p[5]<<16) | ((uvlong)p[6]<<8)
| (uvlong)p[7];
}
uintptr
sysexec(va_list list)
{
union {
struct {
Exec;
uvlong hdr[1];
} ehdr;
char buf[256];
} u;
char line[256];
char *progarg[32+1];
volatile char *args, *elem, *file0;
char **argv, **argp, **argp0;
char *a, *e, *charp, *file;
int i, n, indir;
ulong magic, ssize, nargs, nbytes;
uintptr t, d, b, entry, text, data, bss, bssend, tstk, align;
Segment *s, *ts;
Image *img;
Tos *tos;
Chan *tc;
Fgrp *f;
args = elem = nil;
file0 = va_arg(list, char*);
validaddr((uintptr)file0, 1, 0);
argp0 = va_arg(list, char**);
evenaddr((uintptr)argp0);
validaddr((uintptr)argp0, 2*BY2WD, 0);
if(*argp0 == nil)
error(Ebadarg);
file0 = validnamedup(file0, 1);
if(waserror()){
free(file0);
free(elem);
free(args);
/* Disaster after commit */
if(up->seg[SSEG] == nil)
pexit(up->errstr, 1);
s = up->seg[ESEG];
if(s != nil){
putseg(s);
up->seg[ESEG] = nil;
}
nexterror();
}
align = BY2PG-1;
indir = 0;
file = file0;
for(;;){
tc = namec(file, Aopen, OEXEC, 0);
if(waserror()){
cclose(tc);
nexterror();
}
if(!indir)
kstrdup(&elem, up->genbuf);
n = devtab[tc->type]->read(tc, u.buf, sizeof(u.buf), 0);
if(n >= sizeof(Exec)) {
magic = beswal(u.ehdr.magic);
if(magic == AOUT_MAGIC) {
if(magic & HDR_MAGIC) {
if(n < sizeof(u.ehdr))
error(Ebadexec);
entry = beswav(u.ehdr.hdr[0]);
text = UTZERO+sizeof(u.ehdr);
} else {
entry = beswal(u.ehdr.entry);
text = UTZERO+sizeof(Exec);
}
if(entry < text)
error(Ebadexec);
text += beswal(u.ehdr.text);
if(text <= entry || text >= (USTKTOP-USTKSIZE))
error(Ebadexec);
switch(magic){
case S_MAGIC: /* 2MB segment alignment for amd64 */
align = 0x1fffff;
break;
case V_MAGIC: /* 16K segment alignment for mips */
align = 0x3fff;
break;
case R_MAGIC: /* 64K segment alignment for arm64 */
align = 0xffff;
break;
}
break; /* for binary */
}
}
if(indir++)
error(Ebadexec);
/*
* Process #! /bin/sh args ...
*/
memmove(line, u.buf, n);
n = shargs(line, n, progarg, nelem(progarg));
if(n < 1)
error(Ebadexec);
/*
* First arg becomes complete file name
*/
progarg[n++] = file;
progarg[n] = nil;
argp0++;
file = progarg[0];
progarg[0] = elem;
poperror();
cclose(tc);
}
t = (text+align) & ~align;
text -= UTZERO;
data = beswal(u.ehdr.data);
bss = beswal(u.ehdr.bss);
align = BY2PG-1;
d = (t + data + align) & ~align;
bssend = t + data + bss;
b = (bssend + align) & ~align;
if(t >= (USTKTOP-USTKSIZE) || d >= (USTKTOP-USTKSIZE) || b >= (USTKTOP-USTKSIZE))
error(Ebadexec);
/*
* Args: pass 1: count
*/
nbytes = sizeof(Tos); /* hole for profiling clock at top of stack (and more) */
nargs = 0;
if(indir){
argp = progarg;
while(*argp != nil){
a = *argp++;
nbytes += strlen(a) + 1;
nargs++;
}
}
argp = argp0;
while(*argp != nil){
a = *argp++;
if(((uintptr)argp&(BY2PG-1)) < BY2WD)
validaddr((uintptr)argp, BY2WD, 0);
validaddr((uintptr)a, 1, 0);
e = vmemchr(a, 0, USTKSIZE);
if(e == nil)
error(Ebadarg);
nbytes += (e - a) + 1;
if(nbytes >= USTKSIZE)
error(Enovmem);
nargs++;
}
ssize = BY2WD*(nargs+1) + ((nbytes+(BY2WD-1)) & ~(BY2WD-1));
/*
* 8-byte align SP for those (e.g. sparc) that need it.
* execregs() will subtract another 4 bytes for argc.
*/
if(BY2WD == 4 && (ssize+4) & 7)
ssize += 4;
if(PGROUND(ssize) >= USTKSIZE)
error(Enovmem);
/*
* Build the stack segment, putting it in kernel virtual for the moment
*/
qlock(&up->seglock);
if(waserror()){
qunlock(&up->seglock);
nexterror();
}
s = up->seg[SSEG];
do {
tstk = s->base;
if(tstk <= USTKSIZE)
error(Enovmem);
} while((s = isoverlap(tstk-USTKSIZE, USTKSIZE)) != nil);
up->seg[ESEG] = newseg(SG_STACK | SG_NOEXEC, tstk-USTKSIZE, USTKSIZE/BY2PG);
/*
* Args: pass 2: assemble; the pages will be faulted in
*/
tos = (Tos*)(tstk - sizeof(Tos));
tos->cyclefreq = m->cyclefreq;
tos->kcycles = 0;
tos->pcycles = 0;
tos->clock = 0;
argv = (char**)(tstk - ssize);
charp = (char*)(tstk - nbytes);
if(indir)
argp = progarg;
else
argp = argp0;
for(i=0; i<nargs; i++){
if(indir && *argp==nil) {
indir = 0;
argp = argp0;
}
*argv++ = charp + (USTKTOP-tstk);
a = *argp++;
if(indir)
e = strchr(a, 0);
else {
if(charp >= (char*)tos)
error(Ebadarg);
validaddr((uintptr)a, 1, 0);
e = vmemchr(a, 0, (char*)tos - charp);
if(e == nil)
error(Ebadarg);
}
n = (e - a) + 1;
memmove(charp, a, n);
charp += n;
}
/* copy args; easiest from new process's stack */
a = (char*)(tstk - nbytes);
n = charp - a;
if(n > 128) /* don't waste too much space on huge arg lists */
n = 128;
args = smalloc(n);
memmove(args, a, n);
if(n>0 && args[n-1]!='\0'){
/* make sure last arg is NUL-terminated */
/* put NUL at UTF-8 character boundary */
for(i=n-1; i>0; --i)
if(fullrune(args+i, n-i))
break;
args[i] = 0;
n = i+1;
}
/*
* Committed.
* Free old memory.
* Special segments are maintained across exec
*/
for(i = SSEG; i <= BSEG; i++) {
putseg(up->seg[i]);
/* prevent a second free if we have an error */
up->seg[i] = nil;
}
for(i = ESEG+1; i < NSEG; i++) {
s = up->seg[i];
if(s != nil && (s->type&SG_CEXEC) != 0) {
putseg(s);
up->seg[i] = nil;
}
}
/* Text. Shared. Attaches to cache image if possible */
/* attachimage returns a locked cache image */
img = attachimage(SG_TEXT | SG_RONLY, tc, UTZERO, (t-UTZERO)>>PGSHIFT);
ts = img->s;
up->seg[TSEG] = ts;
ts->flushme = 1;
ts->fstart = 0;
ts->flen = text;
unlock(img);
/* Data. Shared. */
s = newseg(SG_DATA, t, (d-t)>>PGSHIFT);
up->seg[DSEG] = s;
/* Attached by hand */
incref(img);
s->image = img;
s->fstart = ts->fstart+ts->flen;
s->flen = data;
/* BSS. Zero fill on demand */
up->seg[BSEG] = newseg(SG_BSS, d, (b-d)>>PGSHIFT);
/*
* Move the stack
*/
s = up->seg[ESEG];
up->seg[ESEG] = nil;
s->base = USTKTOP-USTKSIZE;
s->top = USTKTOP;
relocateseg(s, USTKTOP-tstk);
up->seg[SSEG] = s;
qunlock(&up->seglock);
poperror(); /* seglock */
/*
* Close on exec
*/
if((f = up->fgrp) != nil) {
for(i=0; i<=f->maxfd; i++)
fdclose(i, CCEXEC);
}
/*
* '/' processes are higher priority (hack to make /ip more responsive).
*/
if(devtab[tc->type]->dc == L'/')
up->basepri = PriRoot;
up->priority = up->basepri;
poperror(); /* tc */
cclose(tc);
poperror(); /* file0 */
free(file0);
qlock(&up->debug);
free(up->text);
up->text = elem;
free(up->args);
up->args = args;
up->nargs = n;
up->setargs = 0;
freenotes(up);
freenote(up->lastnote);
up->lastnote = nil;
up->notify = nil;
up->notified = 0;
up->privatemem = 0;
up->noswap = 0;
up->pcycles = -up->kentry;
procsetup(up);
qunlock(&up->debug);
up->errbuf0[0] = '\0';
up->errbuf1[0] = '\0';
/*
* At this point, the mmu contains info about the old address
* space and needs to be flushed
*/
flushmmu();
if(up->hang)
up->procctl = Proc_stopme;
return execregs(entry, ssize, nargs);
}
int
return0(void*)
{
return 0;
}
uintptr
syssleep(va_list list)
{
long ms;
ms = va_arg(list, long);
if(ms <= 0) {
if (up->edf != nil && (up->edf->flags & Admitted))
edfyield();
else
yield();
} else {
tsleep(&up->sleep, return0, 0, ms);
}
return 0;
}
uintptr
sysalarm(va_list list)
{
return procalarm(va_arg(list, ulong));
}
uintptr
sysexits(va_list list)
{
char *status;
char *inval = "invalid exit string";
char buf[ERRMAX];
status = va_arg(list, char*);
if(status != nil){
if(waserror())
status = inval;
else{
validaddr((uintptr)status, 1, 0);
if(vmemchr(status, 0, ERRMAX) == nil){
memmove(buf, status, ERRMAX);
buf[ERRMAX-1] = 0;
status = buf;
}
poperror();
}
}
pexit(status, 1);
return 0; /* not reached */
}
uintptr
sys_wait(va_list list)
{
ulong pid;
Waitmsg w;
OWaitmsg *ow;
ow = va_arg(list, OWaitmsg*);
if(ow == nil)
pid = pwait(nil);
else {
validaddr((uintptr)ow, sizeof(OWaitmsg), 1);
evenaddr((uintptr)ow);
pid = pwait(&w);
}
if(ow != nil){
readnum(0, ow->pid, NUMSIZE, w.pid, NUMSIZE);
readnum(0, ow->time+TUser*NUMSIZE, NUMSIZE, w.time[TUser], NUMSIZE);
readnum(0, ow->time+TSys*NUMSIZE, NUMSIZE, w.time[TSys], NUMSIZE);
readnum(0, ow->time+TReal*NUMSIZE, NUMSIZE, w.time[TReal], NUMSIZE);
strncpy(ow->msg, w.msg, sizeof(ow->msg)-1);
ow->msg[sizeof(ow->msg)-1] = '\0';
}
return pid;
}
uintptr
sysawait(va_list list)
{
char *p;
Waitmsg w;
uint n;
p = va_arg(list, char*);
n = va_arg(list, uint);
validaddr((uintptr)p, n, 1);
pwait(&w);
return (uintptr)snprint(p, n, "%d %lud %lud %lud %q",
w.pid,
w.time[TUser], w.time[TSys], w.time[TReal],
w.msg);
}
void
werrstr(char *fmt, ...)
{
va_list va;
if(up == nil)
return;
va_start(va, fmt);
vseprint(up->syserrstr, up->syserrstr+ERRMAX, fmt, va);
va_end(va);
}
static int
generrstr(char *buf, uint nbuf)
{
char *err;
if(nbuf == 0)
error(Ebadarg);
if(nbuf > ERRMAX)
nbuf = ERRMAX;
validaddr((uintptr)buf, nbuf, 1);
err = up->errstr;
utfecpy(err, err+nbuf, buf);
utfecpy(buf, buf+nbuf, up->syserrstr);
up->errstr = up->syserrstr;
up->syserrstr = err;
return 0;
}
uintptr
syserrstr(va_list list)
{
char *buf;
uint len;
buf = va_arg(list, char*);
len = va_arg(list, uint);
return (uintptr)generrstr(buf, len);
}
/* compatibility for old binaries */
uintptr
sys_errstr(va_list list)
{
return (uintptr)generrstr(va_arg(list, char*), 64);
}
uintptr
sysnotify(va_list list)
{
int (*f)(void*, char*);
f = va_arg(list, void*);
if(f != nil)
validaddr((uintptr)f, sizeof(void*), 0);
up->notify = f;
return 0;
}
uintptr
sysnoted(va_list list)
{
if(va_arg(list, int) != NRSTR && !up->notified)
error(Egreg);
return 0;
}
uintptr
syssegbrk(va_list list)
{
int i;
uintptr addr;
Segment *s;
addr = va_arg(list, uintptr);
for(i = 0; i < NSEG; i++) {
s = up->seg[i];
if(s == nil || addr < s->base || addr >= s->top)
continue;
switch(s->type&SG_TYPE) {
case SG_TEXT:
case SG_DATA:
case SG_STACK:
case SG_PHYSICAL:
case SG_FIXED:
case SG_STICKY:
error(Ebadarg);
default:
return ibrk(va_arg(list, uintptr), i);
}
}
error(Ebadarg);
return 0; /* not reached */
}
uintptr
syssegattach(va_list list)
{
int attr;
char *name;
uintptr va;
ulong len;
attr = va_arg(list, int);
name = va_arg(list, char*);
va = va_arg(list, uintptr);
len = va_arg(list, ulong);
validaddr((uintptr)name, 1, 0);
name = validnamedup(name, 1);
if(waserror()){
free(name);
nexterror();
}
va = segattach(attr, name, va, len);
free(name);
poperror();
return va;
}
uintptr
syssegdetach(va_list list)
{
int i;
uintptr addr;
Segment *s;
addr = va_arg(list, uintptr);
qlock(&up->seglock);
if(waserror()){
qunlock(&up->seglock);
nexterror();
}
s = nil;
for(i = 0; i < NSEG; i++)
if((s = up->seg[i]) != nil) {
qlock(s);
if((addr >= s->base && addr < s->top) ||
(s->top == s->base && addr == s->base))
goto found;
qunlock(s);
}
error(Ebadarg);
found:
/*
* Check we are not detaching the initial stack segment.
*/
if(s == up->seg[SSEG]){
qunlock(s);
error(Ebadarg);
}
up->seg[i] = nil;
qunlock(s);
putseg(s);
qunlock(&up->seglock);
poperror();
/* Ensure we flush any entries from the lost segment */
flushmmu();
return 0;
}
uintptr
syssegfree(va_list list)
{
Segment *s;
uintptr from, to;
from = va_arg(list, uintptr);
to = va_arg(list, ulong);
to += from;
if(to < from)
error(Ebadarg);
s = seg(up, from, 1);
if(s == nil)
error(Ebadarg);
to &= ~(BY2PG-1);
from = PGROUND(from);
if(from >= to) {
qunlock(s);
return 0;
}
if(to > s->top) {
qunlock(s);
error(Ebadarg);
}
mfreeseg(s, from, (to - from) / BY2PG);
qunlock(s);
flushmmu();
return 0;
}
/* For binary compatibility */
uintptr
sysbrk_(va_list list)
{
return ibrk(va_arg(list, uintptr), BSEG);
}
uintptr
sysrendezvous(va_list list)
{
uintptr tag, val, new;
Proc *p, **l;
tag = va_arg(list, uintptr);
new = va_arg(list, uintptr);
l = &REND(up->rgrp, tag);
lock(up->rgrp);
for(p = *l; p != nil; p = p->rendhash) {
if(p->rendtag == tag) {
*l = p->rendhash;
val = p->rendval;
p->rendval = new;
unlock(up->rgrp);
ready(p);
return val;
}
l = &p->rendhash;
}
/* Going to sleep here */
up->rendtag = tag;
up->rendval = new;
up->rendhash = *l;
*l = up;
up->state = Rendezvous;
unlock(up->rgrp);
sched();
return up->rendval;
}
/*
* The implementation of semaphores is complicated by needing
* to avoid rescheduling in syssemrelease, so that it is safe
* to call from real-time processes. This means syssemrelease
* cannot acquire any qlocks, only spin locks.
*
* Semacquire and semrelease must both manipulate the semaphore
* wait list. Lock-free linked lists only exist in theory, not
* in practice, so the wait list is protected by a spin lock.
*
* The semaphore value *addr is stored in user memory, so it
* cannot be read or written while holding spin locks.
*
* Thus, we can access the list only when holding the lock, and
* we can access the semaphore only when not holding the lock.
* This makes things interesting. Note that sleep's condition function
* is called while holding two locks - r and up->rlock - so it cannot
* access the semaphore value either.
*
* An acquirer announces its intention to try for the semaphore
* by putting a Sema structure onto the wait list and then
* setting Sema.waiting. After one last check of semaphore,
* the acquirer sleeps until Sema.waiting==0. A releaser of n
* must wake up n acquirers who have Sema.waiting set. It does
* this by clearing Sema.waiting and then calling wakeup.
*
* There are three interesting races here.
* The first is that in this particular sleep/wakeup usage, a single
* wakeup can rouse a process from two consecutive sleeps!
* The ordering is:
*
* (a) set Sema.waiting = 1
* (a) call sleep
* (b) set Sema.waiting = 0
* (a) check Sema.waiting inside sleep, return w/o sleeping
* (a) try for semaphore, fail
* (a) set Sema.waiting = 1
* (a) call sleep
* (b) call wakeup(a)
* (a) wake up again
*
* This is okay - semacquire will just go around the loop
* again. It does mean that at the top of the for(;;) loop in
* semacquire, phore.waiting might already be set to 1.
*
* The second is that a releaser might wake an acquirer who is
* interrupted before he can acquire the lock. Since
* release(n) issues only n wakeup calls -- only n can be used
* anyway -- if the interrupted process is not going to use his
* wakeup call he must pass it on to another acquirer.
*
* The third race is similar to the second but more subtle. An
* acquirer sets waiting=1 and then does a final canacquire()
* before going to sleep. The opposite order would result in
* missing wakeups that happen between canacquire and
* waiting=1. (In fact, the whole point of Sema.waiting is to
* avoid missing wakeups between canacquire() and sleep().) But
* there can be spurious wakeups between a successful
* canacquire() and the following semdequeue(). This wakeup is
* not useful to the acquirer, since he has already acquired
* the semaphore. Like in the previous case, though, the
* acquirer must pass the wakeup call along.
*
* This is all rather subtle. The code below has been verified
* with the spin model /sys/src/9/port/semaphore.p. The
* original code anticipated the second race but not the first
* or third, which were caught only with spin. The first race
* is mentioned in /sys/doc/sleep.ps, but I'd forgotten about it.
* It was lucky that my abstract model of sleep/wakeup still managed
* to preserve that behavior.
*
* I remain slightly concerned about memory coherence
* outside of locks. The spin model does not take
* queued processor writes into account so we have to
* think hard. The only variables accessed outside locks
* are the semaphore value itself and the boolean flag
* Sema.waiting. The value is only accessed with cmpswap,
* whose job description includes doing the right thing as
* far as memory coherence across processors. That leaves
* Sema.waiting. To handle it, we call coherence() before each
* read and after each write. - rsc
*/
/* Add semaphore p with addr a to list in seg. */
static void
semqueue(Segment *s, long *a, Sema *p)
{
memset(p, 0, sizeof *p);
p->addr = a;
lock(&s->sema); /* uses s->sema.Rendez.Lock, but no one else is */
p->next = &s->sema;
p->prev = s->sema.prev;
p->next->prev = p;
p->prev->next = p;
unlock(&s->sema);
}
/* Remove semaphore p from list in seg. */
static void
semdequeue(Segment *s, Sema *p)
{
lock(&s->sema);
p->next->prev = p->prev;
p->prev->next = p->next;
unlock(&s->sema);
}
/* Wake up n waiters with addr a on list in seg. */
static void
semwakeup(Segment *s, long *a, long n)
{
Sema *p;
lock(&s->sema);
for(p=s->sema.next; p!=&s->sema && n>0; p=p->next){
if(p->addr == a && p->waiting){
p->waiting = 0;
coherence();
wakeup(p);
n--;
}
}
unlock(&s->sema);
}
/* Add delta to semaphore and wake up waiters as appropriate. */
static long
semrelease(Segment *s, long *addr, long delta)
{
long value;
do
value = *addr;
while(!cmpswap(addr, value, value+delta));
semwakeup(s, addr, delta);
return value+delta;
}
/* Try to acquire semaphore using compare-and-swap */
static int
canacquire(long *addr)
{
long value;
while((value=*addr) > 0)
if(cmpswap(addr, value, value-1))
return 1;
return 0;
}
/* Should we wake up? */
static int
semawoke(void *p)
{
coherence();
return !((Sema*)p)->waiting;
}
/* Acquire semaphore (subtract 1). */
static int
semacquire(Segment *s, long *addr, int block)
{
int acquired;
Sema phore;
if(canacquire(addr))
return 1;
if(!block)
return 0;
acquired = 0;
semqueue(s, addr, &phore);
for(;;){
phore.waiting = 1;
coherence();
if(canacquire(addr)){
acquired = 1;
break;
}
if(waserror())
break;
sleep(&phore, semawoke, &phore);
poperror();
}
semdequeue(s, &phore);
coherence(); /* not strictly necessary due to lock in semdequeue */
if(!phore.waiting)
semwakeup(s, addr, 1);
if(!acquired)
nexterror();
return 1;
}
/* Acquire semaphore or time-out */
static int
tsemacquire(Segment *s, long *addr, ulong ms)
{
int acquired, timedout;
ulong t;
Sema phore;
if(canacquire(addr))
return 1;
if(ms == 0)
return 0;
acquired = timedout = 0;
semqueue(s, addr, &phore);
for(;;){
phore.waiting = 1;
coherence();
if(canacquire(addr)){
acquired = 1;
break;
}
if(waserror())
break;
t = MACHP(0)->ticks;
tsleep(&phore, semawoke, &phore, ms);
t = TK2MS(MACHP(0)->ticks - t);
poperror();
if(t >= ms){
timedout = 1;
break;
}
ms -= t;
}
semdequeue(s, &phore);
coherence(); /* not strictly necessary due to lock in semdequeue */
if(!phore.waiting)
semwakeup(s, addr, 1);
if(timedout)
return 0;
if(!acquired)
nexterror();
return 1;
}
uintptr
syssemacquire(va_list list)
{
int block;
long *addr;
Segment *s;
addr = va_arg(list, long*);
block = va_arg(list, int);
evenaddr((uintptr)addr);
s = seg(up, (uintptr)addr, 0);
if(s == nil || (s->type&SG_RONLY) != 0 || (uintptr)addr+sizeof(long) > s->top){
validaddr((uintptr)addr, sizeof(long), 1);
error(Ebadarg);
}
if(*addr < 0)
error(Ebadarg);
return (uintptr)semacquire(s, addr, block);
}
uintptr
systsemacquire(va_list list)
{
long *addr;
ulong ms;
Segment *s;
addr = va_arg(list, long*);
ms = va_arg(list, ulong);
evenaddr((uintptr)addr);
s = seg(up, (uintptr)addr, 0);
if(s == nil || (s->type&SG_RONLY) != 0 || (uintptr)addr+sizeof(long) > s->top){
validaddr((uintptr)addr, sizeof(long), 1);
error(Ebadarg);
}
if(*addr < 0)
error(Ebadarg);
return (uintptr)tsemacquire(s, addr, ms);
}
uintptr
syssemrelease(va_list list)
{
long *addr, delta;
Segment *s;
addr = va_arg(list, long*);
delta = va_arg(list, long);
evenaddr((uintptr)addr);
s = seg(up, (uintptr)addr, 0);
if(s == nil || (s->type&SG_RONLY) != 0 || (uintptr)addr+sizeof(long) > s->top){
validaddr((uintptr)addr, sizeof(long), 1);
error(Ebadarg);
}
/* delta == 0 is a no-op, not a release */
if(delta < 0 || *addr < 0)
error(Ebadarg);
return (uintptr)semrelease(s, addr, delta);
}
/* For binary compatibility */
uintptr
sys_nsec(va_list list)
{
vlong *v;
/* return in register on 64bit machine */
if(sizeof(uintptr) == sizeof(vlong)){
USED(list);
return (uintptr)todget(nil);
}
v = va_arg(list, vlong*);
evenaddr((uintptr)v);
validaddr((uintptr)v, sizeof(vlong), 1);
*v = todget(nil);
return 0;
}