ref: a98f911a178d3cc044d612e5ad96ee487bd887c8
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>
extern void checkpages(void);
extern void checkpagerefs(void);
uintptr
sysr1(va_list)
{
if(!iseve())
error(Eperm);
checkpagerefs();
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;
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);
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 noattach */
up->pgrp->noattach = opg->noattach;
closepgrp(opg);
}
if(flag & RFNOMNT)
up->pgrp->noattach = 1;
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)
up->noteid = pidalloc(0);
return 0;
}
p = newproc();
p->scallnr = up->scallnr;
p->s = up->s;
p->nerrlab = 0;
p->slash = up->slash;
p->dot = up->dot;
incref(p->dot);
memmove(p->note, up->note, sizeof(p->note));
p->privatemem = up->privatemem;
p->noswap = up->noswap;
p->nnote = up->nnote;
p->notified = 0;
p->lastnote = up->lastnote;
p->notify = up->notify;
p->ureg = up->ureg;
p->dbgreg = 0;
/* Abort the child process on error */
if(waserror()){
p->kp = 1;
kprocchild(p, abortion, 0);
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 noattach */
p->pgrp->noattach = up->pgrp->noattach;
}
else {
p->pgrp = up->pgrp;
incref(p->pgrp);
}
if(flag & RFNOMNT)
p->pgrp->noattach = 1;
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);
}
p->hang = up->hang;
p->procmode = up->procmode;
if(up->procctl == Proc_tracesyscall)
p->procctl = Proc_tracesyscall;
poperror(); /* abortion */
/* 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);
p->parent = up;
if((flag&RFNOWAIT) == 0){
p->parentpid = up->pid;
lock(&up->exl);
up->nchild++;
unlock(&up->exl);
}
if((flag&RFNOTEG) == 0)
p->noteid = up->noteid;
pid = p->pid;
memset(p->time, 0, sizeof(p->time));
p->time[TReal] = MACHP(0)->ticks;
kstrdup(&p->text, up->text);
kstrdup(&p->user, up->user);
procfork(p);
/*
* 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)
procwired(p, wm->machno);
ready(p);
sched();
return pid;
}
static int
shargs(char *s, int n, char **ap)
{
int i;
s += 2;
n -= 2; /* skip #! */
for(i=0;; i++){
if(i >= n)
return 0;
if(s[i]=='\n')
break;
}
s[i] = 0;
i = 0;
for(;;) {
while(*s==' ' || *s=='\t')
s++;
if(*s == 0)
break;
ap[i++] = s++;
while(*s && *s!=' ' && *s!='\t')
s++;
if(*s == 0)
break;
*s++ = 0;
}
ap[i] = nil;
return i;
}
static ulong
l2be(long l)
{
uchar *cp;
cp = (uchar*)&l;
return (cp[0]<<24) | (cp[1]<<16) | (cp[2]<<8) | cp[3];
}
uintptr
sysexec(va_list list)
{
Segment *s, *ts;
int i;
Chan *tc;
char **argv, **argp, **argp0;
char *a, *e, *charp, *args, *file, *file0;
char *progarg[sizeof(Exec)/2+1], *elem, progelem[64];
ulong magic, ssize, nargs, nbytes, n;
uintptr t, d, b, entry, bssend, text, data, bss, tstk, align;
int indir;
Exec exec;
char line[sizeof(Exec)];
Fgrp *f;
Image *img;
Tos *tos;
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;
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, &exec, sizeof(Exec), 0);
if(n <= 2)
error(Ebadexec);
magic = l2be(exec.magic);
if(n == sizeof(Exec) && (magic == AOUT_MAGIC)){
entry = l2be(exec.entry);
text = l2be(exec.text);
if(magic & HDR_MAGIC)
text += 8;
switch(magic){
case S_MAGIC: /* 2MB segment alignment for amd64 */
align = 0x200000;
break;
case V_MAGIC: /* 16K segment alignment for mips */
align = 0x4000;
break;
}
if(text >= (USTKTOP-USTKSIZE)-(UTZERO+sizeof(Exec))
|| entry < UTZERO+sizeof(Exec)
|| entry >= UTZERO+sizeof(Exec)+text)
error(Ebadexec);
break; /* for binary */
}
/*
* Process #! /bin/sh args ...
*/
memmove(line, &exec, n);
if(indir || line[0]!='#' || line[1]!='!')
error(Ebadexec);
n = shargs(line, n, progarg);
if(n < 1)
error(Ebadexec);
indir = 1;
/*
* First arg becomes complete file name
*/
progarg[n++] = file;
progarg[n] = nil;
argp0++;
file = progarg[0];
if(strlen(elem) >= sizeof progelem)
error(Ebadexec);
strcpy(progelem, elem);
progarg[0] = progelem;
poperror();
cclose(tc);
}
data = l2be(exec.data);
bss = l2be(exec.bss);
align--;
t = (UTZERO+sizeof(Exec)+text+align) & ~align;
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(up, tstk-USTKSIZE, USTKSIZE)) != nil);
up->seg[ESEG] = newseg(SG_STACK, 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 {
validaddr((uintptr)a, 1, 0);
e = vmemchr(a, 0, (char*)tstk - 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;
}
}
/*
* Close on exec
*/
if((f = up->fgrp) != nil) {
for(i=0; i<=f->maxfd; i++)
fdclose(i, CCEXEC);
}
/* 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 = sizeof(Exec)+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 */
/*
* '/' 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;
up->nnote = 0;
up->notify = 0;
up->notified = 0;
up->privatemem = 0;
up->noswap = 0;
procsetup(up);
qunlock(&up->debug);
/*
* 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 {
if(ms < TK2MS(1))
ms = TK2MS(1);
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 tmp[ERRMAX];
if(nbuf == 0)
error(Ebadarg);
validaddr((uintptr)buf, nbuf, 1);
if(nbuf > sizeof tmp)
nbuf = sizeof tmp;
memmove(tmp, buf, nbuf);
/* make sure it's NUL-terminated */
tmp[nbuf-1] = '\0';
memmove(buf, up->syserrstr, nbuf);
buf[nbuf-1] = '\0';
memmove(up->syserrstr, tmp, nbuf);
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:
error(Ebadarg);
default:
return (uintptr)ibrk(va_arg(list, uintptr), i);
}
}
error(Ebadarg);
return 0; /* not reached */
}
uintptr
syssegattach(va_list list)
{
ulong attr;
char *name;
uintptr va;
ulong len;
attr = va_arg(list, ulong);
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(up, 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 (uintptr)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, elms;
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 = m->ticks;
tsleep(&phore, semawoke, &phore, ms);
elms = TK2MS(m->ticks - t);
poperror();
if(elms >= ms){
timedout = 1;
break;
}
ms -= elms;
}
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;
}