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8234773: Fix ThreadsSMRSupport::_bootstrap_list #1921

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8234773: Fix ThreadsSMRSupport::_bootstrap_list #1921

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d3240a1
fix
kimbarrett Jan 3, 2021
1a6cbcb
Make ThreadsList constructor explicit.
kimbarrett Jan 4, 2021
39b183e
Merge branch 'master' into bootstrap_thread_list
kimbarrett Jan 8, 2021
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28 changes: 21 additions & 7 deletions src/hotspot/share/runtime/threadSMR.cpp
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Expand Up @@ -76,7 +76,7 @@ volatile uint ThreadsSMRSupport::_deleted_thread_time_max = 0;
volatile uint ThreadsSMRSupport::_deleted_thread_times = 0;

// The bootstrap list is empty and cannot be freed.
ThreadsList ThreadsSMRSupport::_bootstrap_list = ThreadsList(0);
ThreadsList ThreadsSMRSupport::_bootstrap_list{0};
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To anyone unfamiliar with the details of direct initialization this is very obscure. I don't know how to interpret this given a ThreadsList has four member fields. Wouldn't {} have the same effect of implicitly zero-initializing all the fields?
Stylistically I find this syntax jarring, an equals sign in there would look far more natural to me. :(


// This is the VM's current "threads list" and it contains all of
// the JavaThreads the VM considers to be alive at this moment in
Expand Down Expand Up @@ -585,18 +585,32 @@ void SafeThreadsListPtr::verify_hazard_ptr_scanned() {
#endif
}

// 'entries + 1' so we always have at least one entry.
// Shared singleton data for all ThreadsList(0) instances.
// Used by _bootstrap_list to avoid static init time heap allocation.
// No real entries, just the final NULL terminator.
static JavaThread* const empty_threads_list_data[1] = {};

// Result has 'entries + 1' elements, with the last being the NULL terminator.
static JavaThread* const* make_threads_list_data(int entries) {
if (entries == 0) {
return empty_threads_list_data;
}
JavaThread** data = NEW_C_HEAP_ARRAY(JavaThread*, entries + 1, mtThread);
data[entries] = NULL; // Make sure the final entry is NULL.
return data;
}

ThreadsList::ThreadsList(int entries) :
_length(entries),
_next_list(NULL),
_threads(NEW_C_HEAP_ARRAY(JavaThread*, entries + 1, mtThread)),
_threads(make_threads_list_data(entries)),
_nested_handle_cnt(0)
{
*(JavaThread**)(_threads + entries) = NULL; // Make sure the extra entry is NULL.
}
{}

ThreadsList::~ThreadsList() {
FREE_C_HEAP_ARRAY(JavaThread*, _threads);
if (_threads != empty_threads_list_data) {
FREE_C_HEAP_ARRAY(JavaThread*, _threads);
}
}

// Add a JavaThread to a ThreadsList. The returned ThreadsList is a
Expand Down
4 changes: 3 additions & 1 deletion src/hotspot/share/runtime/threadSMR.hpp
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Expand Up @@ -172,6 +172,8 @@ class ThreadsList : public CHeapObj<mtThread> {
JavaThread *const *const _threads;
volatile intx _nested_handle_cnt;

NONCOPYABLE(ThreadsList);

template <class T>
void threads_do_dispatch(T *cl, JavaThread *const thread) const;

Expand All @@ -185,7 +187,7 @@ class ThreadsList : public CHeapObj<mtThread> {
static ThreadsList* remove_thread(ThreadsList* list, JavaThread* java_thread);

public:
ThreadsList(int entries);
explicit ThreadsList(int entries);
~ThreadsList();

template <class T>
Expand Down