/* -----------------------------------------------------------------------------
Evaluate a THUNK_SELECTOR if possible.
p points to a THUNK_SELECTOR that we want to evaluate. The
result of "evaluating" it will be evacuated and a pointer to the
to-space closure will be returned.
If the THUNK_SELECTOR could not be evaluated (its selectee is still
a THUNK, for example), then the THUNK_SELECTOR itself will be
evacuated.
-------------------------------------------------------------------------- */
static void
unchain_thunk_selectors(StgSelector *p, StgClosure *val)
{
StgSelector *prev;
prev = NULL;
while (p)
{
ASSERT(p->header.info == &stg_WHITEHOLE_info);
// val must be in to-space. Not always: when we recursively
// invoke eval_thunk_selector(), the recursive calls will not
// evacuate the value (because we want to select on the value,
// not evacuate it), so in this case val is in from-space.
// ASSERT(!HEAP_ALLOCED_GC(val) || Bdescr((P_)val)->gen_no > N || (Bdescr((P_)val)->flags & BF_EVACUATED));
prev = (StgSelector*)((StgClosure *)p)->payload[0];
// Update the THUNK_SELECTOR with an indirection to the
// value. The value is still in from-space at this stage.
//
// (old note: Why not do upd_evacuee(q,p)? Because we have an
// invariant that an EVACUATED closure always points to an
// object in the same or an older generation (required by
// the short-cut test in the EVACUATED case, below).
if ((StgClosure *)p == val) {
// must be a loop; just leave a BLACKHOLE in place. This
// can happen when we have a chain of selectors that
// eventually loops back on itself. We can't leave an
// indirection pointing to itself, and we want the program
// to deadlock if it ever enters this closure, so
// BLACKHOLE is correct.
// XXX we do not have BLACKHOLEs any more; replace with
// a THUNK_SELECTOR again. This will go into a loop if it is
// entered, and should result in a NonTermination exception.
((StgThunk *)p)->payload[0] = val;
write_barrier();
SET_INFO((StgClosure *)p, &stg_sel_0_upd_info);
} else {
((StgInd *)p)->indirectee = val;
write_barrier();
SET_INFO((StgClosure *)p, &stg_IND_info);
}
// For the purposes of LDV profiling, we have created an
// indirection.
LDV_RECORD_CREATE(p);
p = prev;
}
}
/* -----------------------------------------------------------------------------
* 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;
}
}
