static void
eval_thunk_selector (StgClosure **q, StgSelector * p, rtsBool evac)
// NB. for legacy reasons, p & q are swapped around :(
{
nat field;
StgInfoTable *info;
StgWord info_ptr;
StgClosure *selectee;
StgSelector *prev_thunk_selector;
bdescr *bd;
StgClosure *val;
prev_thunk_selector = NULL;
// this is a chain of THUNK_SELECTORs that we are going to update
// to point to the value of the current THUNK_SELECTOR. Each
// closure on the chain is a WHITEHOLE, and points to the next in the
// chain with payload[0].
selector_chain:
bd = Bdescr((StgPtr)p);
if (HEAP_ALLOCED_GC(p)) {
// If the THUNK_SELECTOR is in to-space or in a generation that we
// are not collecting, then bale out early. We won't be able to
// save any space in any case, and updating with an indirection is
// trickier in a non-collected gen: we would have to update the
// mutable list.
if (bd->flags & BF_EVACUATED) {
unchain_thunk_selectors(prev_thunk_selector, (StgClosure *)p);
*q = (StgClosure *)p;
// shortcut, behave as for: if (evac) evacuate(q);
if (evac && bd->gen_no < gct->evac_gen_no) {
gct->failed_to_evac = rtsTrue;
TICK_GC_FAILED_PROMOTION();
}
return;
}
// we don't update THUNK_SELECTORS in the compacted
// generation, because compaction does not remove the INDs
// that result, this causes confusion later
// (scavenge_mark_stack doesn't deal with IND). BEWARE! This
// bit is very tricky to get right. If you make changes
// around here, test by compiling stage 3 with +RTS -c -RTS.
if (bd->flags & BF_MARKED) {
// must call evacuate() to mark this closure if evac==rtsTrue
*q = (StgClosure *)p;
if (evac) evacuate(q);
unchain_thunk_selectors(prev_thunk_selector, (StgClosure *)p);
return;
}
}
// WHITEHOLE the selector thunk, since it is now under evaluation.
// This is important to stop us going into an infinite loop if
// this selector thunk eventually refers to itself.
#if defined(THREADED_RTS)
// In threaded mode, we'll use WHITEHOLE to lock the selector
// thunk while we evaluate it.
{
do {
info_ptr = xchg((StgPtr)&p->header.info, (W_)&stg_WHITEHOLE_info);
} while (info_ptr == (W_)&stg_WHITEHOLE_info);
// make sure someone else didn't get here first...
if (IS_FORWARDING_PTR(info_ptr) ||
INFO_PTR_TO_STRUCT((StgInfoTable *)info_ptr)->type != THUNK_SELECTOR) {
// v. tricky now. The THUNK_SELECTOR has been evacuated
// by another thread, and is now either a forwarding ptr or IND.
// We need to extract ourselves from the current situation
// as cleanly as possible.
// - unlock the closure
// - update *q, we may have done *some* evaluation
// - if evac, we need to call evacuate(), because we
// need the write-barrier stuff.
// - undo the chain we've built to point to p.
SET_INFO((StgClosure *)p, (const StgInfoTable *)info_ptr);
*q = (StgClosure *)p;
if (evac) evacuate(q);
unchain_thunk_selectors(prev_thunk_selector, (StgClosure *)p);
return;
}
}
#else
// Save the real info pointer (NOTE: not the same as get_itbl()).
info_ptr = (StgWord)p->header.info;
SET_INFO((StgClosure *)p,&stg_WHITEHOLE_info);
#endif
field = INFO_PTR_TO_STRUCT((StgInfoTable *)info_ptr)->layout.selector_offset;
// The selectee might be a constructor closure,
// so we untag the pointer.
selectee = UNTAG_CLOSURE(p->selectee);
selector_loop:
// selectee now points to the closure that we're trying to select
// a field from. It may or may not be in to-space: we try not to
// end up in to-space, but it's impractical to avoid it in
// general. The compacting GC scatters to-space pointers in
// from-space during marking, for example. We rely on the property
// that evacuate() doesn't mind if it gets passed a to-space pointer.
info = (StgInfoTable*)selectee->header.info;
if (IS_FORWARDING_PTR(info)) {
// We don't follow pointers into to-space; the constructor
// has already been evacuated, so we won't save any space
// leaks by evaluating this selector thunk anyhow.
goto bale_out;
}
info = INFO_PTR_TO_STRUCT(info);
switch (info->type) {
case WHITEHOLE:
goto bale_out; // about to be evacuated by another thread (or a loop).
case CONSTR:
case CONSTR_1_0:
case CONSTR_0_1:
case CONSTR_2_0:
case CONSTR_1_1:
case CONSTR_0_2:
case CONSTR_STATIC:
case CONSTR_NOCAF_STATIC:
{
// check that the size is in range
ASSERT(field < (StgWord32)(info->layout.payload.ptrs +
info->layout.payload.nptrs));
// Select the right field from the constructor
val = selectee->payload[field];
#ifdef PROFILING
// For the purposes of LDV profiling, we have destroyed
// the original selector thunk, p.
if (era > 0) {
// Only modify the info pointer when LDV profiling is
// enabled. Note that this is incompatible with parallel GC,
// because it would allow other threads to start evaluating
// the same selector thunk.
SET_INFO((StgClosure*)p, (StgInfoTable *)info_ptr);
OVERWRITING_CLOSURE((StgClosure*)p);
SET_INFO((StgClosure*)p, &stg_WHITEHOLE_info);
}
#endif
// the closure in val is now the "value" of the
// THUNK_SELECTOR in p. However, val may itself be a
// THUNK_SELECTOR, in which case we want to continue
// evaluating until we find the real value, and then
// update the whole chain to point to the value.
val_loop:
info_ptr = (StgWord)UNTAG_CLOSURE(val)->header.info;
if (!IS_FORWARDING_PTR(info_ptr))
{
info = INFO_PTR_TO_STRUCT((StgInfoTable *)info_ptr);
switch (info->type) {
case IND:
case IND_STATIC:
val = ((StgInd *)val)->indirectee;
goto val_loop;
case THUNK_SELECTOR:
((StgClosure*)p)->payload[0] = (StgClosure *)prev_thunk_selector;
prev_thunk_selector = p;
p = (StgSelector*)val;
goto selector_chain;
default:
break;
}
}
((StgClosure*)p)->payload[0] = (StgClosure *)prev_thunk_selector;
prev_thunk_selector = p;
*q = val;
// update the other selectors in the chain *before*
// evacuating the value. This is necessary in the case
// where the value turns out to be one of the selectors
// in the chain (i.e. we have a loop), and evacuating it
// would corrupt the chain.
unchain_thunk_selectors(prev_thunk_selector, val);
// evacuate() cannot recurse through
// eval_thunk_selector(), because we know val is not
// a THUNK_SELECTOR.
if (evac) evacuate(q);
return;
}
case IND:
case IND_STATIC:
// Again, we might need to untag a constructor.
selectee = UNTAG_CLOSURE( ((StgInd *)selectee)->indirectee );
goto selector_loop;
case BLACKHOLE:
{
StgClosure *r;
const StgInfoTable *i;
r = ((StgInd*)selectee)->indirectee;
// establish whether this BH has been updated, and is now an
// indirection, as in evacuate().
if (GET_CLOSURE_TAG(r) == 0) {
i = r->header.info;
if (IS_FORWARDING_PTR(i)) {
r = (StgClosure *)UN_FORWARDING_PTR(i);
i = r->header.info;
}
if (i == &stg_TSO_info
|| i == &stg_WHITEHOLE_info
|| i == &stg_BLOCKING_QUEUE_CLEAN_info
|| i == &stg_BLOCKING_QUEUE_DIRTY_info) {
goto bale_out;
}
ASSERT(i != &stg_IND_info);
}
selectee = UNTAG_CLOSURE( ((StgInd *)selectee)->indirectee );
goto selector_loop;
}
case THUNK_SELECTOR:
{
StgClosure *val;
// recursively evaluate this selector. We don't want to
// recurse indefinitely, so we impose a depth bound.
if (gct->thunk_selector_depth >= MAX_THUNK_SELECTOR_DEPTH) {
goto bale_out;
}
gct->thunk_selector_depth++;
// rtsFalse says "don't evacuate the result". It will,
// however, update any THUNK_SELECTORs that are evaluated
// along the way.
eval_thunk_selector(&val, (StgSelector*)selectee, rtsFalse);
gct->thunk_selector_depth--;
// did we actually manage to evaluate it?
if (val == selectee) goto bale_out;
// Of course this pointer might be tagged...
selectee = UNTAG_CLOSURE(val);
goto selector_loop;
}
case AP:
case AP_STACK:
case THUNK:
case THUNK_1_0:
case THUNK_0_1:
case THUNK_2_0:
case THUNK_1_1:
case THUNK_0_2:
case THUNK_STATIC:
// not evaluated yet
goto bale_out;
default:
barf("eval_thunk_selector: strange selectee %d",
(int)(info->type));
}
bale_out:
// We didn't manage to evaluate this thunk; restore the old info
// pointer. But don't forget: we still need to evacuate the thunk itself.
SET_INFO((StgClosure *)p, (const StgInfoTable *)info_ptr);
// THREADED_RTS: we just unlocked the thunk, so another thread
// might get in and update it. copy() will lock it again and
// check whether it was updated in the meantime.
*q = (StgClosure *)p;
if (evac) {
copy(q,(const StgInfoTable *)info_ptr,(StgClosure *)p,THUNK_SELECTOR_sizeW(),bd->dest_no);
}
unchain_thunk_selectors(prev_thunk_selector, *q);
return;
}
/* -----------------------------------------------------------------------------
* Pausing a thread
*
* We have to prepare for GC - this means doing lazy black holing
* here. We also take the opportunity to do stack squeezing if it's
* turned on.
* -------------------------------------------------------------------------- */
void
threadPaused(Capability *cap, StgTSO *tso)
{
StgClosure *frame;
const StgRetInfoTable *info;
const StgInfoTable *bh_info;
const StgInfoTable *cur_bh_info USED_IF_THREADS;
StgClosure *bh;
StgPtr stack_end;
uint32_t words_to_squeeze = 0;
uint32_t weight = 0;
uint32_t weight_pending = 0;
bool prev_was_update_frame = false;
StgWord heuristic_says_squeeze;
// Check to see whether we have threads waiting to raise
// exceptions, and we're not blocking exceptions, or are blocked
// interruptibly. This is important; if a thread is running with
// TSO_BLOCKEX and becomes blocked interruptibly, this is the only
// place we ensure that the blocked_exceptions get a chance.
maybePerformBlockedException (cap, tso);
if (tso->what_next == ThreadKilled) { return; }
// NB. Updatable thunks *must* be blackholed, either by eager blackholing or
// lazy blackholing. See Note [upd-black-hole] in sm/Scav.c.
stack_end = tso->stackobj->stack + tso->stackobj->stack_size;
frame = (StgClosure *)tso->stackobj->sp;
while ((P_)frame < stack_end) {
info = get_ret_itbl(frame);
switch (info->i.type) {
case UPDATE_FRAME:
// If we've already marked this frame, then stop here.
if (frame->header.info == (StgInfoTable *)&stg_marked_upd_frame_info) {
if (prev_was_update_frame) {
words_to_squeeze += sizeofW(StgUpdateFrame);
weight += weight_pending;
weight_pending = 0;
}
goto end;
}
SET_INFO(frame, (StgInfoTable *)&stg_marked_upd_frame_info);
bh = ((StgUpdateFrame *)frame)->updatee;
bh_info = bh->header.info;
#if defined(THREADED_RTS)
retry:
#endif
// Note [suspend duplicate work]
//
// If the info table is a WHITEHOLE or a BLACKHOLE, then
// another thread has claimed it (via the SET_INFO()
// below), or is in the process of doing so. In that case
// we want to suspend the work that the current thread has
// done on this thunk and wait until the other thread has
// finished.
//
// If eager blackholing is taking place, it could be the
// case that the blackhole points to the current
// TSO. e.g.:
//
// this thread other thread
// --------------------------------------------------------
// c->indirectee = other_tso;
// c->header.info = EAGER_BH
// threadPaused():
// c->header.info = WHITEHOLE
// c->indirectee = other_tso
// c->indirectee = this_tso;
// c->header.info = EAGER_BH
// c->header.info = BLACKHOLE
// threadPaused()
// *** c->header.info is now BLACKHOLE,
// c->indirectee points to this_tso
//
// So in this case do *not* suspend the work of the
// current thread, because the current thread will become
// deadlocked on itself. See #5226 for an instance of
// this bug.
//
// Note that great care is required when entering computations
// suspended by this mechanism. See Note [AP_STACKs must be eagerly
// blackholed] for details.
if (((bh_info == &stg_BLACKHOLE_info)
&& ((StgInd*)bh)->indirectee != (StgClosure*)tso)
|| (bh_info == &stg_WHITEHOLE_info))
{
debugTrace(DEBUG_squeeze,
"suspending duplicate work: %ld words of stack",
(long)((StgPtr)frame - tso->stackobj->sp));
// If this closure is already an indirection, then
// suspend the computation up to this point.
// NB. check raiseAsync() to see what happens when
// we're in a loop (#2783).
suspendComputation(cap,tso,(StgUpdateFrame*)frame);
// Now drop the update frame, and arrange to return
// the value to the frame underneath:
tso->stackobj->sp = (StgPtr)frame + sizeofW(StgUpdateFrame) - 2;
tso->stackobj->sp[1] = (StgWord)bh;
ASSERT(bh->header.info != &stg_TSO_info);
tso->stackobj->sp[0] = (W_)&stg_enter_info;
// And continue with threadPaused; there might be
// yet more computation to suspend.
frame = (StgClosure *)(tso->stackobj->sp + 2);
prev_was_update_frame = false;
continue;
}
// We should never have made it here in the event of blackholes that
// we already own; they should have been marked when we blackholed
// them and consequently we should have stopped our stack walk
// above.
ASSERT(!((bh_info == &stg_BLACKHOLE_info)
&& (((StgInd*)bh)->indirectee == (StgClosure*)tso)));
// zero out the slop so that the sanity checker can tell
// where the next closure is.
OVERWRITING_CLOSURE(bh);
// an EAGER_BLACKHOLE or CAF_BLACKHOLE gets turned into a
// BLACKHOLE here.
#if defined(THREADED_RTS)
// first we turn it into a WHITEHOLE to claim it, and if
// successful we write our TSO and then the BLACKHOLE info pointer.
cur_bh_info = (const StgInfoTable *)
cas((StgVolatilePtr)&bh->header.info,
(StgWord)bh_info,
(StgWord)&stg_WHITEHOLE_info);
if (cur_bh_info != bh_info) {
bh_info = cur_bh_info;
goto retry;
}
#endif
// The payload of the BLACKHOLE points to the TSO
((StgInd *)bh)->indirectee = (StgClosure *)tso;
write_barrier();
SET_INFO(bh,&stg_BLACKHOLE_info);
// .. and we need a write barrier, since we just mutated the closure:
recordClosureMutated(cap,bh);
// We pretend that bh has just been created.
LDV_RECORD_CREATE(bh);
frame = (StgClosure *) ((StgUpdateFrame *)frame + 1);
if (prev_was_update_frame) {
words_to_squeeze += sizeofW(StgUpdateFrame);
weight += weight_pending;
weight_pending = 0;
}
prev_was_update_frame = true;
break;
case UNDERFLOW_FRAME:
case STOP_FRAME:
goto end;
// normal stack frames; do nothing except advance the pointer
default:
{
uint32_t frame_size = stack_frame_sizeW(frame);
weight_pending += frame_size;
frame = (StgClosure *)((StgPtr)frame + frame_size);
prev_was_update_frame = false;
}
}
}
end:
// Should we squeeze or not? Arbitrary heuristic: we squeeze if
// the number of words we have to shift down is less than the
// number of stack words we squeeze away by doing so.
// The threshold was bumped from 5 to 8 as a result of #2797
heuristic_says_squeeze = ((weight <= 8 && words_to_squeeze > 0)
|| weight < words_to_squeeze);
debugTrace(DEBUG_squeeze,
"words_to_squeeze: %d, weight: %d, squeeze: %s",
words_to_squeeze, weight,
heuristic_says_squeeze ? "YES" : "NO");
if (RtsFlags.GcFlags.squeezeUpdFrames == true &&
heuristic_says_squeeze) {
stackSqueeze(cap, tso, (StgPtr)frame);
tso->flags |= TSO_SQUEEZED;
// This flag tells threadStackOverflow() that the stack was
// squeezed, because it may not need to be expanded.
} else {
tso->flags &= ~TSO_SQUEEZED;
}
}
