/src/9vx/a/sysproc.c
C | 1139 lines | 820 code | 134 blank | 185 comment | 170 complexity | a9e667cec4e768e6a22873c037c8b9d6 MD5 | raw file
Possible License(s): MPL-2.0-no-copyleft-exception, LGPL-2.1
- #define WANT_M
- #include "u.h"
- #include "tos.h"
- #include "lib.h"
- #include "mem.h"
- #include "dat.h"
- #include "fns.h"
- #include "error.h"
- #include "a.out.h"
- int shargs(char*, int, char**);
- extern void checkpages(void);
- extern void checkpagerefs(void);
- long
- sysr1(uint32 *x)
- {
- vx32sysr1();
- return 0;
- }
- long
- sysrfork(uint32 *arg)
- {
- Proc *p;
- int n, i;
- Fgrp *ofg;
- Pgrp *opg;
- Rgrp *org;
- Egrp *oeg;
- ulong pid, flag;
- Mach *wm;
- flag = arg[0];
- /* 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.ref = 1;
- if(flag & RFENVG)
- envcpy(up->egrp, oeg);
- closeegrp(oeg);
- }
- if(flag & RFNOTEG)
- up->noteid = incref(¬eidalloc);
- return 0;
- }
- p = newproc();
- p->fpsave = up->fpsave;
- p->scallnr = up->scallnr;
- p->s = up->s;
- p->nerrlab = 0;
- p->slash = up->slash;
- p->dot = up->dot;
- incref(&p->dot->ref);
- 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;
- /* 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])
- 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->ref);
- }
- /* 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->ref);
- }
- if(flag & RFNOMNT)
- up->pgrp->noattach = 1;
- if(flag & RFREND)
- p->rgrp = newrgrp();
- else {
- incref(&up->rgrp->ref);
- p->rgrp = up->rgrp;
- }
- /* Environment group */
- if(flag & (RFENVG|RFCENVG)) {
- p->egrp = smalloc(sizeof(Egrp));
- p->egrp->ref.ref = 1;
- if(flag & RFENVG)
- envcpy(p->egrp, up->egrp);
- }
- else {
- p->egrp = up->egrp;
- incref(&p->egrp->ref);
- }
- p->hang = up->hang;
- p->procmode = up->procmode;
- /* 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;
- p->parentpid = up->pid;
- if(flag&RFNOWAIT)
- p->parentpid = 0;
- else {
- lock(&up->exl);
- up->nchild++;
- unlock(&up->exl);
- }
- if((flag&RFNOTEG) == 0)
- p->noteid = up->noteid;
- p->fpstate = up->fpstate;
- pid = p->pid;
- memset(p->time, 0, sizeof(p->time));
- p->time[TReal] = msec();
- kstrdup(&p->text, up->text);
- kstrdup(&p->user, up->user);
- /*
- * 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 uint32
- l2be(uint32 l)
- {
- uchar *cp;
- cp = (uchar*)&l;
- return (cp[0]<<24) | (cp[1]<<16) | (cp[2]<<8) | cp[3];
- }
- static char Echanged[] = "exec arguments changed underfoot";
- long
- sysexec(uint32 *arg)
- {
- char *volatile elem, *volatile file, *ufile;
- Chan *volatile tc;
- /*
- * Open the file, remembering the final element and the full name.
- */
- file = nil;
- elem = nil;
- tc = nil;
- if(waserror()){
- if(file)
- free(file);
- if(elem)
- free(elem);
- if(tc)
- cclose(tc);
- nexterror();
- }
- ufile = uvalidaddr(arg[0], 1, 0);
- file = validnamedup(ufile, 1);
- tc = namec(file, Aopen, OEXEC, 0);
- kstrdup((char**)&elem, up->genbuf);
- /*
- * Read the header. If it's a #!, fill in progarg[] with info and repeat.
- */
- int i, n, nprogarg;
- char *progarg[sizeof(Exec)/2+1];
- char *prog, *p;
- char line[sizeof(Exec)+1];
- Exec exec;
- nprogarg = 0;
- n = devtab[tc->type]->read(tc, &exec, sizeof(Exec), 0);
- if(n < 2)
- error(Ebadexec);
- p = (char*)&exec;
- if(p[0] == '#' && p[1] == '!'){
- memmove(line, p, n);
- nprogarg = shargs(line, n, progarg);
- if(nprogarg == 0)
- error(Ebadexec);
-
- /* The original file becomes an extra arg after #! line */
- progarg[nprogarg++] = file;
-
- /*
- * Take the #! $0 as a file to open, and replace
- * $0 with the original path's name.
- */
- prog = progarg[0];
- progarg[0] = elem;
- cclose(tc);
- tc = nil; /* in case namec errors out */
- tc = namec(prog, Aopen, OEXEC, 0);
- n = devtab[tc->type]->read(tc, &exec, sizeof(Exec), 0);
- if(n < 2)
- error(Ebadexec);
- }
- /*
- * #! has had its chance, now we need a real binary
- */
- uint32 magic, entry, text, etext, data, edata, bss, ebss;
- magic = l2be(exec.magic);
- if(n != sizeof(Exec) || l2be(exec.magic) != AOUT_MAGIC)
- error(Ebadexec);
- entry = l2be(exec.entry);
- text = l2be(exec.text);
- data = l2be(exec.data);
- bss = l2be(exec.bss);
- etext = ROUND(UTZERO+sizeof(Exec)+text, BY2PG);
- edata = ROUND(etext + data, BY2PG);
- ebss = ROUND(etext + data + bss, BY2PG);
-
- //iprint("entry %#lux text %#lux data %#lux bss %#lux\n", entry, text, data, bss);
- //iprint("etext %#lux edata %#lux ebss %#lux\n", etext, edata, ebss);
- if(entry < UTZERO+sizeof(Exec) || entry >= UTZERO+sizeof(Exec)+text)
- error(Ebadexec);
-
- /* many overflow possibilities */
- if(text >= USTKTOP || data >= USTKTOP || bss >= USTKTOP
- || etext >= USTKTOP || edata >= USTKTOP || ebss >= USTKTOP
- || etext >= USTKTOP || edata < etext || ebss < edata)
- error(Ebadexec);
- /*
- * Copy argv into new stack segment temporarily mapped elsewhere.
- * Be careful: multithreaded program could be changing argv during this.
- * Pass 1: count number of arguments, string bytes.
- */
- int nargv, strbytes;
- uint32 argp, ssize, spage;
- strbytes = 0;
- for(i=0; i<nprogarg; i++)
- strbytes += strlen(progarg[i]) + 1;
- argp = arg[1];
- for(nargv=0;; nargv++, argp += BY2WD){
- uint32 a;
- char *str;
- a = *(uint32*)uvalidaddr(argp, BY2WD, 0);
- if(a == 0)
- break;
- str = uvalidaddr(a, 1, 0);
- n = ((char*)vmemchr(str, 0, 0x7FFFFFFF) - str) + 1;
- if(nprogarg > 0 && nargv == 0)
- continue; /* going to skip argv[0] on #! */
- strbytes += n;
- }
- if(nargv == 0)
- error("exec missing argv");
- /*
- * Skip over argv[0] if using #!. Waited until now so that
- * string would still be checked for validity during loop.
- */
- if(nprogarg > 0){
- nargv--;
- arg[1] += BY2WD;
- }
- ssize = BY2WD*((nprogarg+nargv)+1) + ROUND(strbytes, BY2WD) + sizeof(Tos);
- /*
- * 8-byte align SP for those (e.g. sparc) that need it.
- * execregs() will subtract another 4 bytes for argc.
- */
- if((ssize+4) & 7)
- ssize += 4;
- spage = (ssize+(BY2PG-1)) >> PGSHIFT;
- /*
- * Pass 2: build the stack segment, being careful not to assume
- * that the counts from pass 1 are still valid.
- */
- if(spage > TSTKSIZ)
- error(Enovmem);
- qlock(&up->seglock);
- if(waserror()){
- if(up->seg[ESEG]){
- putseg(up->seg[ESEG]);
- up->seg[ESEG] = nil;
- }
- qunlock(&up->seglock);
- nexterror();
- }
- up->seg[ESEG] = newseg(SG_STACK, TSTKTOP-USTKSIZE, USTKSIZE/BY2PG);
- flushmmu(); // Needed for Plan 9 VX XXX really?
- /*
- * Top-of-stack structure.
- */
- uchar *uzero;
- uzero = up->pmmu.uzero;
- Tos *tos;
- uint32 utos;
- utos = USTKTOP - sizeof(Tos);
- tos = (Tos*)(uzero + utos + TSTKTOP - USTKTOP);
- tos->cyclefreq = m->cyclefreq;
- cycles((uvlong*)&tos->pcycles);
- tos->pcycles = -tos->pcycles;
- tos->kcycles = tos->pcycles;
- tos->clock = 0;
- /*
- * Argument pointers and strings, together.
- */
- char *bp, *ep;
- uint32 *targp;
- uint32 ustrp, uargp;
- ustrp = utos - ROUND(strbytes, BY2WD);
- uargp = ustrp - BY2WD*((nprogarg+nargv)+1);
- bp = (char*)(uzero + ustrp + TSTKTOP - USTKTOP);
- ep = bp + strbytes;
- p = bp;
- targp = (uint32*)(uzero + uargp + TSTKTOP - USTKTOP);
-
- /* #! args are trusted */
- for(i=0; i<nprogarg; i++){
- n = strlen(progarg[i]) + 1;
- if(n > ep - p)
- error(Echanged);
- memmove(p, progarg[i], n);
- p += n;
- *targp++ = ustrp;
- ustrp += n;
- }
-
- /* the rest are not */
- argp = arg[1];
- for(i=0; i<nargv; i++){
- uint32 a;
- char *str;
-
- a = *(uint32*)uvalidaddr(argp, BY2WD, 0);
- argp += BY2WD;
-
- str = uvalidaddr(a, 1, 0);
- n = ((char*)vmemchr(str, 0, 0x7FFFFFFF) - str) + 1;
- if(n > ep - p)
- error(Echanged);
- memmove(p, str, n);
- p += n;
- *targp++ = ustrp;
- ustrp += n;
- }
- if(*(uint32*)uvalidaddr(argp, BY2WD, 0) != 0)
- error(Echanged);
- *targp = 0;
- /*
- * But wait, there's more: prepare an arg copy for up->args
- * using the copy we just made in the temporary segment.
- */
- char *args;
- int nargs;
- n = p - bp; /* includes NUL on last arg, so must be > 0 */
- if(n <= 0) /* nprogarg+nargv > 0; checked above */
- error(Egreg);
- if(n > 128)
- n = 128;
- args = smalloc(n);
- if(waserror()){
- free(args);
- nexterror();
- }
- memmove(args, bp, n);
- /* find beginning of UTF character boundary to place final NUL */
- while(n > 0 && (args[n-1]&0xC0) == 0x80)
- n--;
- args[n-1] = '\0';
- nargs = n;
- /*
- * Now we're ready to commit.
- */
- free(up->text);
- up->text = elem;
- free(up->args);
- up->args = args;
- up->nargs = n;
- elem = nil;
- poperror(); /* args */
- /*
- * Free old memory. Special segments maintained across exec.
- */
- Segment *s;
- for(i = SSEG; i <= BSEG; i++) {
- putseg(up->seg[i]);
- up->seg[i] = nil; /* in case of error */
- }
- for(i = BSEG+1; i< NSEG; i++) {
- s = up->seg[i];
- if(s && (s->type&SG_CEXEC)) {
- putseg(s);
- up->seg[i] = nil;
- }
- }
-
- /*
- * Close on exec
- */
- Fgrp *f;
- f = up->fgrp;
- for(i=0; i<=f->maxfd; i++)
- fdclose(i, CCEXEC);
- /* Text. Shared. Attaches to cache image if possible */
- /* attachimage returns a locked cache image */
- Image *img;
- Segment *ts;
- img = attachimage(SG_TEXT|SG_RONLY, tc, UTZERO, (etext-UTZERO)>>PGSHIFT);
- ts = img->s;
- up->seg[TSEG] = ts;
- ts->flushme = 1;
- ts->fstart = 0;
- ts->flen = sizeof(Exec)+text;
- unlock(&img->ref.lk);
- /* Data. Shared. */
- s = newseg(SG_DATA, etext, (edata-etext)>>PGSHIFT);
- up->seg[DSEG] = s;
- /* Attached by hand */
- incref(&img->ref);
- s->image = img;
- s->fstart = ts->fstart+ts->flen;
- s->flen = data;
- /* BSS. Zero fill on demand */
- up->seg[BSEG] = newseg(SG_BSS, edata, (ebss-edata)>>PGSHIFT);
- /*
- * Move the stack
- */
- s = up->seg[ESEG];
- up->seg[ESEG] = 0;
- up->seg[SSEG] = s;
- qunlock(&up->seglock);
- poperror(); /* seglock */
- s->base = USTKTOP-USTKSIZE;
- s->top = USTKTOP;
- relocateseg(s, USTKTOP-TSTKTOP);
- /*
- * '/' 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, elem, file */
- cclose(tc);
- free(file);
- // elem is now up->text
- /*
- * At this point, the mmu contains info about the old address
- * space and needs to be flushed
- */
- flushmmu();
- qlock(&up->debug);
- up->nnote = 0;
- up->notify = 0;
- up->notified = 0;
- up->privatemem = 0;
- procsetup(up);
- qunlock(&up->debug);
- if(up->hang)
- up->procctl = Proc_stopme;
- return execregs(entry, USTKTOP - uargp, nprogarg+nargv);
- }
- int
- shargs(char *s, int n, char **ap)
- {
- int i;
- s += 2;
- n -= 2; /* skip #! */
- for(i=0; s[i]!='\n'; i++)
- if(i == n-1)
- return 0;
- s[i] = 0;
- *ap = 0;
- i = 0;
- for(;;) {
- while(*s==' ' || *s=='\t')
- s++;
- if(*s == 0)
- break;
- i++;
- *ap++ = s;
- *ap = 0;
- while(*s && *s!=' ' && *s!='\t')
- s++;
- if(*s == 0)
- break;
- else
- *s++ = 0;
- }
- return i;
- }
- int
- return0(void *v)
- {
- return 0;
- }
- long
- syssleep(uint32 *arg)
- {
- int n;
- n = arg[0];
- if(n <= 0) {
- yield();
- return 0;
- }
- if(n < TK2MS(1))
- n = TK2MS(1);
- tsleep(&up->sleep, return0, 0, n);
- return 0;
- }
- long
- sysalarm(uint32 *arg)
- {
- return procalarm(arg[0]);
- }
- long
- sysexits(uint32 *arg)
- {
- char *status;
- char *inval = "invalid exit string";
- char buf[ERRMAX];
- if(arg[0]){
- if(waserror())
- status = inval;
- else{
- status = uvalidaddr(arg[0], 1, 0);
- if(vmemchr(status, 0, ERRMAX) == 0){
- memmove(buf, status, ERRMAX);
- buf[ERRMAX-1] = 0;
- status = buf;
- }
- poperror();
- }
- }else
- status = nil;
- pexit(status, 1);
- return 0; /* not reached */
- }
- long
- sys_wait(uint32 *arg)
- {
- int pid;
- Waitmsg w;
- OWaitmsg *ow;
- if(arg[0] == 0)
- return pwait(nil);
- ow = uvalidaddr(arg[0], sizeof(OWaitmsg), 1);
- evenaddr(arg[0]);
- pid = pwait(&w);
- if(pid >= 0){
- 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));
- ow->msg[sizeof(ow->msg)-1] = '\0';
- }
- return pid;
- }
- long
- sysawait(uint32 *arg)
- {
- int i;
- int pid;
- Waitmsg w;
- uint32 n;
- char *buf;
- n = arg[1];
- buf = uvalidaddr(arg[0], n, 1);
- pid = pwait(&w);
- if(pid < 0)
- return -1;
- i = snprint(buf, n, "%d %lud %lud %lud %q",
- w.pid,
- w.time[TUser], w.time[TSys], w.time[TReal],
- w.msg);
- return i;
- }
- 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 long
- generrstr(uint32 addr, uint nbuf)
- {
- char tmp[ERRMAX];
- char *buf;
- if(nbuf == 0)
- error(Ebadarg);
- buf = uvalidaddr(addr, 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;
- }
- long
- syserrstr(uint32 *arg)
- {
- return generrstr(arg[0], arg[1]);
- }
- /* compatibility for old binaries */
- long
- sys_errstr(uint32 *arg)
- {
- return generrstr(arg[0], 64);
- }
- long
- sysnotify(uint32 *arg)
- {
- if(arg[0] != 0)
- uvalidaddr(arg[0], 1, 0);
- up->notify = arg[0]; /* checked again when used */
- return 0;
- }
- long
- sysnoted(uint32 *arg)
- {
- if(arg[0]!=NRSTR && !up->notified)
- error(Egreg);
- return 0;
- }
- long
- syssegbrk(uint32 *arg)
- {
- int i;
- uint32 addr;
- Segment *s;
- addr = arg[0];
- for(i = 0; i < NSEG; i++) {
- s = up->seg[i];
- if(s == 0 || addr < s->base || addr >= s->top)
- continue;
- switch(s->type&SG_TYPE) {
- case SG_TEXT:
- case SG_DATA:
- case SG_STACK:
- error(Ebadarg);
- default:
- return ibrk(arg[1], i);
- }
- }
- error(Ebadarg);
- return 0; /* not reached */
- }
- long
- syssegattach(uint32 *arg)
- {
- return segattach(up, arg[0], uvalidaddr(arg[1], 1, 0), arg[2], arg[3]);
- }
- long
- syssegdetach(uint32 *arg)
- {
- int i;
- uint32 addr;
- Segment *s;
- qlock(&up->seglock);
- if(waserror()){
- qunlock(&up->seglock);
- nexterror();
- }
- s = 0;
- addr = arg[0];
- for(i = 0; i < NSEG; i++)
- if((s = up->seg[i])) {
- qlock(&s->lk);
- if((addr >= s->base && addr < s->top) ||
- (s->top == s->base && addr == s->base))
- goto found;
- qunlock(&s->lk);
- }
- error(Ebadarg);
- found:
- /*
- * Check we are not detaching the initial stack segment.
- */
- if(s == up->seg[SSEG]){
- qunlock(&s->lk);
- error(Ebadarg);
- }
- up->seg[i] = 0;
- qunlock(&s->lk);
- putseg(s);
- qunlock(&up->seglock);
- poperror();
- /* Ensure we flush any entries from the lost segment */
- flushmmu();
- return 0;
- }
- long
- syssegfree(uint32 *arg)
- {
- Segment *s;
- uint32 from, to;
- from = arg[0];
- s = seg(up, from, 1);
- if(s == nil)
- error(Ebadarg);
- to = (from + arg[1]) & ~(BY2PG-1);
- from = PGROUND(from);
- if(to > s->top) {
- qunlock(&s->lk);
- error(Ebadarg);
- }
- mfreeseg(s, from, (to - from) / BY2PG);
- qunlock(&s->lk);
- flushmmu();
- return 0;
- }
- /* For binary compatibility */
- long
- sysbrk_(uint32 *arg)
- {
- return ibrk(arg[0], BSEG);
- }
- long
- sysrendezvous(uint32 *arg)
- {
- uintptr tag, val;
- Proc *p, **l;
- tag = arg[0];
- l = &REND(up->rgrp, tag);
- up->rendval = ~(uintptr)0;
- lock(&up->rgrp->ref.lk);
- for(p = *l; p; p = p->rendhash) {
- if(p->rendtag == tag) {
- *l = p->rendhash;
- val = p->rendval;
- p->rendval = arg[1];
- while(p->mach != 0)
- ;
- ready(p);
- unlock(&up->rgrp->ref.lk);
- return val;
- }
- l = &p->rendhash;
- }
- /* Going to sleep here */
- up->rendtag = tag;
- up->rendval = arg[1];
- up->rendhash = *l;
- *l = up;
- up->state = Rendezvous;
- unlock(&up->rgrp->ref.lk);
- 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.rendez.lk); /* 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.rendez.lk);
- }
- /* Remove semaphore p from list in seg. */
- static void
- semdequeue(Segment *s, Sema *p)
- {
- lock(&s->sema.rendez.lk);
- p->next->prev = p->prev;
- p->prev->next = p->next;
- unlock(&s->sema.rendez.lk);
- }
- /* 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.rendez.lk);
- 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->rendez);
- n--;
- }
- }
- unlock(&s->sema.rendez.lk);
- }
- /* 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.rendez, 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;
- }
- long
- syssemacquire(uint32 *arg)
- {
- int block;
- long *addr;
- Segment *s;
- addr = uvalidaddr(arg[0], sizeof(long), 1);
- evenaddr(arg[0]);
- block = arg[1];
-
- if((s = seg(up, arg[0], 0)) == nil)
- error(Ebadarg);
- if(*addr < 0)
- error(Ebadarg);
- return semacquire(s, addr, block);
- }
- long
- syssemrelease(uint32 *arg)
- {
- long *addr, delta;
- Segment *s;
- addr = uvalidaddr(arg[0], sizeof(long), 1);
- evenaddr(arg[0]);
- delta = arg[1];
- if((s = seg(up, arg[0], 0)) == nil)
- error(Ebadarg);
- if(delta < 0 || *addr < 0)
- error(Ebadarg);
- return semrelease(s, addr, arg[1]);
- }