gecko-dev/js/ref/jslock.c

/* -*- Mode: C; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 *
 * The contents of this file are subject to the Netscape Public License
 * Version 1.0 (the "NPL"); you may not use this file except in
 * compliance with the NPL.  You may obtain a copy of the NPL at
 * http://www.mozilla.org/NPL/
 *
 * Software distributed under the NPL is distributed on an "AS IS" basis,
 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the NPL
 * for the specific language governing rights and limitations under the
 * NPL.
 *
 * The Initial Developer of this code under the NPL is Netscape
 * Communications Corporation.  Portions created by Netscape are
 * Copyright (C) 1998 Netscape Communications Corporation.  All Rights
 * Reserved.
 */

#ifdef JS_THREADSAFE

/*
 * JS locking stubs.
 */
#include "jsstddef.h"
#include <stdlib.h>
#include "jspubtd.h"
#include "prthread.h"
#include "pratom.h"
#include "prassert.h"
#include "jscntxt.h"
#include "jsscope.h"
#include "jspubtd.h"
#include "jslock.h"

static PRLock *_global_lock;

static void
js_LockGlobal()
{
  PR_Lock(_global_lock);
}

static void
js_UnlockGlobal()
{
  PR_Unlock(_global_lock);
}

#define ReadWord(W) (W)
#define AtomicAddBody(P,I)\
    prword n;\
    do {\
	n = ReadWord(*(P));\
    } while (!js_CompareAndSwap(P, n, n + I));

#if defined(_WIN32) && !defined(NSPR_LOCK)
#pragma warning( disable : 4035 )

PR_INLINE int
js_CompareAndSwap(prword *w, prword ov, prword nv)
{
    __asm {
		mov eax,ov
		mov ecx, nv
		mov ebx, w
		lock cmpxchg [ebx], ecx
		sete al
		and eax,1h
	}
}

#elif defined(SOLARIS) && !defined(NSPR_LOCK)

#ifndef ULTRA_SPARC
PR_INLINE prword
js_ReadWord(prword *p)
{
    prword n;
    while ((n = *p) == -1)
	;
    return n;
}

#undef ReadWord
#define ReadWord(W) js_ReadWord(&(W))

static PRLock *_counter_lock;
#define UsingCounterLock 1

#undef AtomicAddBody
#define AtomicAddBody(P,I) \
    PR_Lock(_counter_lock);\
    *(P) += I;\
    PR_Unlock(_counter_lock);

#endif /* !ULTRA_SPARC */

PR_INLINE int
js_CompareAndSwap(prword *w, prword ov, prword nv)
{
#if defined(__GNUC__)
    unsigned int res;
#ifndef ULTRA_SPARC
    PR_ASSERT(nv >= 0);
    asm volatile ("
stbar
swap [%1],%4
1:  tst %4
bneg,a 1b
swap [%1],%4
cmp %2,%4
be,a 2f
swap [%1],%3
swap [%1],%4
ba 3f
mov 0,%0
2:  mov 1,%0
3:"
		  : "=r" (res)
		  : "r" (w), "r" (ov), "r" (nv), "r" (-1));
#else /* ULTRA_SPARC */
    PR_ASSERT(ov != nv);
    asm volatile ("
stbar
cas [%1],%2,%3
cmp %2,%3
be,a 1f
mov 1,%0
mov 0,%0
1:"
		  : "=r" (res)
		  : "r" (w), "r" (ov), "r" (nv));
#endif /* ULTRA_SPARC */
    return (int)res;
#else /* !__GNUC__ */
    extern int compare_and_swap(prword*,prword,prword);
#ifndef ULTRA_SPARC
    PR_ASSERT(nv >= 0);
#else
    PR_ASSERT(ov != nv);
#endif
    return compare_and_swap(w,ov,nv);
#endif
}

#elif defined(AIX) && !defined(NSPR_LOCK)
#include <sys/atomic_op.h>

PR_INLINE int
js_CompareAndSwap(prword *w, prword ov, prword nv)
{
    return !_check_lock(w,ov,nv);
}

#else

static PRLock *_counter_lock;
#define UsingCounterLock 1

#undef AtomicAddBody
#define AtomicAddBody(P,I) \
    PR_Lock(_counter_lock);\
    *(P) += I;\
    PR_Unlock(_counter_lock);

static PRLock *_compare_and_swap_lock;
#define UsingCompareAndSwapLock 1

PR_INLINE int
js_CompareAndSwap(prword *w, prword ov, prword nv)
{
    int res = 0;

    PR_Lock(_compare_and_swap_lock);
    if (*w == ov) {
      *w = nv;
      res = 1;
    }
    PR_Unlock(_compare_and_swap_lock);
    return res;
}

#endif /* arch-tests */

PR_INLINE void
js_AtomicAdd(prword *p, prword i)
{
    AtomicAddBody(p,i);
}

PR_INLINE prword
js_AtomicSet(prword *p, prword n)
{
    prword o;
    do {
	o = ReadWord(*p);
    } while (!js_CompareAndSwap(p,o,n));
    return o;
}

prword
js_CurrentThreadId()
{
    return CurrentThreadId();
}

void
js_NewLock(JSThinLock *p)
{
#ifdef NSPR_LOCK
    p->owner = 0;
    p->fat = (JSFatLock*)JS_NEW_LOCK();
#else
    memset(p, 0, sizeof(JSThinLock));
#endif
}

void
js_DestroyLock(JSThinLock *p)
{
#ifdef NSPR_LOCK
    p->owner = 0xdeadbeef;
    JS_DESTROY_LOCK(((JSLock*)p->fat));
#else
    PR_ASSERT(p->owner == 0);
    PR_ASSERT(p->fat == NULL);
#endif
}

static void js_Dequeue(JSThinLock *);

PR_INLINE jsval
js_GetSlotWhileLocked(JSContext *cx, JSObject *obj, uint32 slot)
{
    jsval v;
#ifndef NSPR_LOCK
    JSScope *scp = (JSScope *)obj->map;
    JSThinLock *p = &scp->lock;
    prword me = cx->thread;
#endif

    PR_ASSERT(obj->slots && slot < obj->map->freeslot);
#ifndef NSPR_LOCK
    PR_ASSERT(me == CurrentThreadId());
    if (js_CompareAndSwap(&p->owner, 0, me)) {
	if (scp == (JSScope *)obj->map) {
	    v = obj->slots[slot];
	    if (!js_CompareAndSwap(&p->owner, me, 0)) {
		scp->count = 1;
		js_UnlockObj(cx,obj);
	    }
	    return v;
	}
	if (!js_CompareAndSwap(&p->owner, me, 0))
	    js_Dequeue(p);
    }
    else if (Thin_RemoveWait(ReadWord(p->owner)) == me) {
	return obj->slots[slot];
    }
#endif
    js_LockObj(cx,obj);
    v = obj->slots[slot];
    js_UnlockObj(cx,obj);
    return v;
}

PR_INLINE void
js_SetSlotWhileLocked(JSContext *cx, JSObject *obj, uint32 slot, jsval v)
{
#ifndef NSPR_LOCK
    JSScope *scp = (JSScope *)obj->map;
    JSThinLock *p = &scp->lock;
    prword me = cx->thread;
#endif

    PR_ASSERT(obj->slots && slot < obj->map->freeslot);
#ifndef NSPR_LOCK
    PR_ASSERT(me == CurrentThreadId());
    if (js_CompareAndSwap(&p->owner, 0, me)) {
	if (scp == (JSScope *)obj->map) {
	    obj->slots[slot] = v;
	    if (!js_CompareAndSwap(&p->owner, me, 0)) {
		scp->count = 1;
		js_UnlockObj(cx,obj);
	    }
	    return;
	}
	if (!js_CompareAndSwap(&p->owner, me, 0))
	    js_Dequeue(p);
    }
    else if (Thin_RemoveWait(ReadWord(p->owner)) == me) {
	obj->slots[slot] = v;
	return;
    }
#endif
    js_LockObj(cx,obj);
    obj->slots[slot] = v;
    js_UnlockObj(cx,obj);
}

static JSFatLock *
mallocFatlock()
{
    JSFatLock *fl = (JSFatLock *)malloc(sizeof(JSFatLock)); /* for now */
    PR_ASSERT(fl);
    fl->susp = 0;
    fl->next = NULL;
    fl->prev = NULL;
    fl->slock = PR_NewLock();
    fl->svar = PR_NewCondVar(fl->slock);
    return fl;
}

static void
freeFatlock(JSFatLock *fl)
{
    PR_DestroyLock(fl->slock);
    PR_DestroyCondVar(fl->svar);
    free(fl);
}

static int
js_SuspendThread(JSThinLock *p)
{
    JSFatLock *fl;
    PRStatus stat;

    while ((fl = (JSFatLock*)js_AtomicSet((prword*)&p->fat,1)) == (JSFatLock*)1) /* busy wait */
	PR_Sleep(PR_INTERVAL_NO_WAIT);
    if (fl == NULL)
	return 1;
    PR_Lock(fl->slock);
    js_AtomicSet((prword*)&p->fat,(prword)fl);
    fl->susp++;
    if (fl->susp < 1) {
	PR_Unlock(fl->slock);
	return 1;
    }
    stat = PR_WaitCondVar(fl->svar,PR_INTERVAL_NO_TIMEOUT);
    if (stat == PR_FAILURE) {
	fl->susp--;
	return 0;
    }
    PR_Unlock(fl->slock);
    return 1;
}

static void
js_ResumeThread(JSThinLock *p)
{
    JSFatLock *fl;
    PRStatus stat;

    while ((fl = (JSFatLock*)js_AtomicSet((prword*)&p->fat,1)) == (JSFatLock*)1)
	PR_Sleep(PR_INTERVAL_NO_WAIT);
    if (fl == NULL)
	return;
    PR_Lock(fl->slock);
    js_AtomicSet((prword*)&p->fat,(prword)fl);
    fl->susp--;
    if (fl->susp < 0) {
	PR_Unlock(fl->slock);
	return;
    }
    stat = PR_NotifyCondVar(fl->svar);
    PR_ASSERT(stat != PR_FAILURE);
    PR_Unlock(fl->slock);
}

static JSFatLock *
listOfFatlocks(int l)
{
    JSFatLock *m;
    JSFatLock *m0;
	int i;

    PR_ASSERT(l>0);
    m0 = m = mallocFatlock();
    for (i=1; i<l; i++) {
		m->next = mallocFatlock();
		m = m->next;
    }
    return m0;
}

static void
deleteListOfFatlocks(JSFatLock *m)
{
    JSFatLock *m0;
    for (; m; m=m0) {
	m0 = m->next;
	freeFatlock(m);
    }
}

static JSFatLockTable _fl_table;

JSFatLock *
allocateFatlock()
{
  JSFatLock *m;

  js_LockGlobal();
  if (_fl_table.free == NULL) {
#ifdef DEBUG
      printf("Ran out of fat locks!\n");
#endif
      _fl_table.free = listOfFatlocks(10);
  }
  m = _fl_table.free;
  _fl_table.free = m->next;
  _fl_table.free->prev = NULL;
  m->susp = 0;
  m->next = _fl_table.taken;
  m->prev = NULL;
  if (_fl_table.taken != NULL)
	_fl_table.taken->prev = m;
  _fl_table.taken = m;
  js_UnlockGlobal();
  return m;
}

void
deallocateFatlock(JSFatLock *m)
{
  if (m == NULL)
	return;
  js_LockGlobal();
  if (m->prev != NULL)
	m->prev->next = m->next;
  if (m->next != NULL)
	m->next->prev = m->prev;
  if (m == _fl_table.taken)
	_fl_table.taken = m->next;
  m->next = _fl_table.free;
  _fl_table.free = m;
  js_UnlockGlobal();
}

JS_PUBLIC_API(int)
js_SetupLocks(int l)
{
  if (l > 10000)       /* l equals number of initially allocated fat locks */
#ifdef DEBUG
    printf("Number %d very large in js_SetupLocks()!\n",l);
#endif
  if (_global_lock)
    return 1;
  _global_lock = PR_NewLock();
  PR_ASSERT(_global_lock);
#ifdef UsingCounterLock
  _counter_lock = PR_NewLock();
  PR_ASSERT(_counter_lock);
#endif
#ifdef UsingCompareAndSwapLock
  _compare_and_swap_lock = PR_NewLock();
  PR_ASSERT(_compare_and_swap_lock);
#endif
  _fl_table.free = listOfFatlocks(l);
  _fl_table.taken = NULL;
  return 1;
}

JS_PUBLIC_API(void)
js_CleanupLocks()
{
  if (_global_lock != NULL) {
    deleteListOfFatlocks(_fl_table.free);
    _fl_table.free = NULL;
    deleteListOfFatlocks(_fl_table.taken);
    _fl_table.taken = NULL;
    PR_DestroyLock(_global_lock);
    _global_lock = NULL;
#ifdef UsingCounterLock
    PR_DestroyLock(_counter_lock);
    _counter_lock = NULL;
#endif
#ifdef UsingCompareAndSwapLock
    PR_DestroyLock(_compare_and_swap_lock);
    _compare_and_swap_lock = NULL;
#endif
  }
}

JS_PUBLIC_API(void)
js_InitContextForLocking(JSContext *cx)
{
	cx->thread = CurrentThreadId();
	PR_ASSERT(Thin_GetWait(cx->thread) == 0);
}

/*

  It is important that emptyFatlock() clears p->fat in the empty case
  while holding p->slock. This serializes the access of p->fat wrt
  js_SuspendThread(), which also requires p->slock.  There would
  otherwise be a race condition as follows.  Thread A is about to
  clear p->fat, having woken up after being suspended in a situation
  where no other thread has yet suspended on p. However, suppose
  thread B is about to suspend on p, having just released the global
  lock, and is currently calling js_SuspendThread(). In the unfortunate
  case where A precedes B ever so slightly, A will think that it
  should clear p->fat (being currently empty but not for long), at the
  same time as B is about to suspend on p->fat.  Now, A deallocates
  p->fat while B is about to suspend on it. Thus, the suspension of B
  is lost and B will not be properly activated.

  Using p->slock as below (and correspondingly in js_SuspendThread()),
  js_SuspendThread() will notice that p->fat is empty, and hence return
  immediately.

  */

int
emptyFatlock(JSThinLock *p)
{
    JSFatLock *fl;
    int i;
    PRLock* lck;

    while ((fl = (JSFatLock*)js_AtomicSet((prword*)&p->fat,1)) == (JSFatLock*)1)
       PR_Sleep(PR_INTERVAL_NO_WAIT);
    if (fl == NULL) {
	js_AtomicSet((prword*)&p->fat,(prword)fl);
	return 1;
    }
    lck = fl->slock;
    PR_Lock(lck);
    i = fl->susp;
    if (i < 1) {
	fl->susp = -1;
	deallocateFatlock(fl);
	fl = NULL;
    }
    js_AtomicSet((prword*)&p->fat,(prword)fl);
    PR_Unlock(lck);
    return i < 1;
}

/*
  Fast locking and unlocking is implemented by delaying the
  allocation of a system lock (fat lock) until contention. As long as
  a locking thread A runs uncontended, the lock is represented solely
  by storing A's identity in the object being locked.

  If another thread B tries to lock the object currently locked by A,
  B is enqueued into a fat lock structure (which might have to be
  allocated and pointed to by the object), and suspended using NSPR
  conditional variables (wait).  A wait bit (Bacon bit) is set in the
  lock word of the object, signalling to A that when releasing the
  lock, B must be dequeued and notified.

  The basic operation of the locking primitives (js_Lock(),
  js_Unlock(), js_Enqueue(), and js_Dequeue()) is
  compare-and-swap. Hence, when locking into p, if
  compare-and-swap(p,NULL,me) succeeds this implies that p is
  unlocked.  Similarly, when unlocking p, if
  compare-and-swap(p,me,NULL) succeeds this implies that p is
  uncontended (no one is waiting because the wait bit is not set).

  Furthermore, when enqueueing (after the compare-and-swap has failed
  to lock the object), p->fat is used to serialize the different
  accesses to the fat lock. The four function thus synchronized are
  js_Enqueue, emptyFatLock, js_SuspendThread, and js_ResumeThread.

  When dequeueing, the lock is released, and one of the threads
  suspended on the lock is notified.  If other threads still are
  waiting, the wait bit is kept (in js_Enqueue), and if not, the fat
  lock is deallocated (in emptyFatlock()).

  p->fat is set to 1 by enqueue and emptyFatlock to signal that the pointer
  is being accessed.

*/

static void
js_Enqueue(JSThinLock *p, prword me)
{
    prword o, n;

    while (1) {
	o = ReadWord(p->owner);
	n = Thin_SetWait(o);
	if (o != 0 && js_CompareAndSwap(&p->owner,o,n)) {
	    JSFatLock* fl;
	    while ((fl = (JSFatLock*)js_AtomicSet((prword*)&p->fat,1)) == (JSFatLock*)1)
		PR_Sleep(PR_INTERVAL_NO_WAIT);
	    if (fl == NULL)
		fl = allocateFatlock();
	    js_AtomicSet((prword*)&p->fat,(prword)fl);
	    js_SuspendThread(p);
	    if (emptyFatlock(p))
		me = Thin_RemoveWait(me);
	    else
		me = Thin_SetWait(me);
	}
	else if (js_CompareAndSwap(&p->owner,0,me)) {
	    return;
	}
    }
}

static void
js_Dequeue(JSThinLock *p)
{
    int o = ReadWord(p->owner);
    PR_ASSERT(Thin_GetWait(o));
    if (!js_CompareAndSwap(&p->owner,o,0)) /* release it */
	PR_ASSERT(0);
    js_ResumeThread(p);
}

PR_INLINE void
js_Lock(JSThinLock *p, prword me)
{
    PR_ASSERT(me == CurrentThreadId());
    if (js_CompareAndSwap(&p->owner, 0, me))
	return;
    if (Thin_RemoveWait(ReadWord(p->owner)) != me)
	js_Enqueue(p, me);
#ifdef DEBUG
    else
	PR_ASSERT(0);
#endif
}

PR_INLINE void
js_Unlock(JSThinLock *p, prword me)
{
    PR_ASSERT(me == CurrentThreadId());
    if (js_CompareAndSwap(&p->owner, me, 0))
	return;
    if (Thin_RemoveWait(ReadWord(p->owner)) == me)
	js_Dequeue(p);
#ifdef DEBUG
    else
	PR_ASSERT(0);
#endif
}

void
js_LockRuntime(JSRuntime *rt)
{
    prword me = CurrentThreadId();
    JSThinLock *p;

    PR_ASSERT(Thin_RemoveWait(ReadWord(rt->rtLock.owner)) != me);
    p = &rt->rtLock;
    JS_LOCK0(p,me);
}

void
js_UnlockRuntime(JSRuntime *rt)
{
    prword me = CurrentThreadId();
    JSThinLock *p;

    PR_ASSERT(Thin_RemoveWait(ReadWord(rt->rtLock.owner)) == me);
    p = &rt->rtLock;
    JS_UNLOCK0(p,me);
}

PR_INLINE void
js_LockScope1(JSContext *cx, JSScope *scope, prword me)
{
    JSThinLock *p;

    if (Thin_RemoveWait(ReadWord(scope->lock.owner)) == me) {
	PR_ASSERT(scope->count > 0);
	scope->count++;
    } else {
	p = &scope->lock;
	JS_LOCK0(p,me);
	PR_ASSERT(scope->count == 0);
	scope->count = 1;
    }
}

void
js_LockScope(JSContext *cx, JSScope *scope)
{
    PR_ASSERT(cx->thread == CurrentThreadId());
    js_LockScope1(cx,scope,cx->thread);
}

void
js_UnlockScope(JSContext *cx, JSScope *scope)
{
    prword me = cx->thread;
    JSThinLock *p;

    PR_ASSERT(scope->count > 0);
    if (Thin_RemoveWait(ReadWord(scope->lock.owner)) != me) {
	PR_ASSERT(0);
	return;
    }
    if (--scope->count == 0) {
	p = &scope->lock;
	JS_UNLOCK0(p,me);
    }
}

void
js_TransferScopeLock(JSContext *cx, JSScope *oldscope, JSScope *newscope)
{
    prword me;
    JSThinLock *p;

    PR_ASSERT(JS_IS_SCOPE_LOCKED(newscope));
    /*
     * If the last reference to oldscope went away, newscope needs no lock
     * state update.
     */
    if (!oldscope) {
	return;
    }
    PR_ASSERT(JS_IS_SCOPE_LOCKED(oldscope));
    /*
     * Transfer oldscope's entry count to newscope, as it will be unlocked
     * now via JS_UNLOCK_OBJ(cx,obj) calls made while we unwind the C stack
     * from the current point (under js_GetMutableScope).
     */
    newscope->count = oldscope->count;
    /*
     * Reset oldscope's lock state so that it is completely unlocked.
     */
    oldscope->count = 0;
    p = &oldscope->lock;
    me = cx->thread;
    JS_UNLOCK0(p,me);
}

void
js_LockObj(JSContext *cx, JSObject *obj)
{
    JSScope *scope;
    prword me = cx->thread;
    PR_ASSERT(me == CurrentThreadId());
    for (;;) {
		scope = (JSScope *) obj->map;
		js_LockScope1(cx, scope, me);

		/* If obj still has this scope, we're done. */
		if (scope == (JSScope *) obj->map)
			return;

		/* Lost a race with a mutator; retry with obj's new scope. */
		js_UnlockScope(cx, scope);
    }
}

void
js_UnlockObj(JSContext *cx, JSObject *obj)
{
    js_UnlockScope(cx, (JSScope *) obj->map);
}

#ifdef DEBUG
JSBool
js_IsRuntimeLocked(JSRuntime *rt)
{
    return CurrentThreadId() == Thin_RemoveWait(ReadWord(rt->rtLock.owner));
}

JSBool
js_IsObjLocked(JSObject *obj)
{
    JSObjectMap *map = obj->map;

    return MAP_IS_NATIVE(map) && CurrentThreadId() == Thin_RemoveWait(ReadWord(((JSScope *)map)->lock.owner));
}

JSBool
js_IsScopeLocked(JSScope *scope)
{
    return CurrentThreadId() == Thin_RemoveWait(ReadWord(scope->lock.owner));
}
#endif

#undef ReadWord
#endif /* JS_THREADSAFE */