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8334890: Missing unconditional cross modifying fence in nmethod entry barriers

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Reviewed-by: aboldtch, kbarrett
This commit is contained in:
Erik Österlund 2024-07-04 10:06:09 +00:00
parent cf1be87279
commit c0604fb823
1 changed files with 4 additions and 20 deletions
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24
src/hotspot/share/gc/shared/barrierSetNMethod.cpp
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@ -178,16 +178,9 @@ int BarrierSetNMethod::nmethod_stub_entry_barrier(address* return_address_ptr) {
nmethod* nm = cb->as_nmethod();
BarrierSetNMethod* bs_nm = BarrierSet::barrier_set()->barrier_set_nmethod();
// Check for disarmed method here to avoid going into DeoptimizeNMethodBarriersALot code
// too often. nmethod_entry_barrier checks for disarmed status itself,
// but we have no visibility into whether the barrier acted or not.
if (!bs_nm->is_armed(nm)) {
return 0;
}
assert(!nm->is_osr_method(), "Should not reach here");
// Called upon first entry after being armed
bool may_enter = bs_nm->nmethod_entry_barrier(nm);
assert(!nm->is_osr_method() || may_enter, "OSR nmethods should always be entrant after migration");
// In case a concurrent thread disarmed the nmethod, we need to ensure the new instructions
// are made visible, by using a cross modify fence. Note that this is synchronous cross modifying
@ -197,11 +190,11 @@ int BarrierSetNMethod::nmethod_stub_entry_barrier(address* return_address_ptr) {
// it can be made conditional on the nmethod_patching_type.
OrderAccess::cross_modify_fence();
// Diagnostic option to force deoptimization 1 in 3 times. It is otherwise
// Diagnostic option to force deoptimization 1 in 10 times. It is otherwise
// a very rare event.
if (DeoptimizeNMethodBarriersALot) {
if (DeoptimizeNMethodBarriersALot && !nm->is_osr_method()) {
static volatile uint32_t counter=0;
if (Atomic::add(&counter, 1u) % 3 == 0) {
if (Atomic::add(&counter, 1u) % 10 == 0) {
may_enter = false;
}
}
@ -214,15 +207,6 @@ int BarrierSetNMethod::nmethod_stub_entry_barrier(address* return_address_ptr) {
}
bool BarrierSetNMethod::nmethod_osr_entry_barrier(nmethod* nm) {
// This check depends on the invariant that all nmethods that are deoptimized / made not entrant
// are NOT disarmed.
// This invariant is important because a method can be deoptimized after the method have been
// resolved / looked up by OSR by another thread. By not deoptimizing them we guarantee that
// a deoptimized method will always hit the barrier and come to the same conclusion - deoptimize
if (!is_armed(nm)) {
return true;
}
assert(nm->is_osr_method(), "Should not reach here");
log_trace(nmethod, barrier)("Running osr nmethod entry barrier: " PTR_FORMAT, p2i(nm));
bool result = nmethod_entry_barrier(nm);