Thank you for both quickly identifying a solution and taking steps to integrate it across the codebase!
TBB is a bit complicated depending on which platform it’s on. You can see the full details here, but tracing back the relevant bits for VTK’s usage:
From Common/Core/SMP/TBB/vtkSMPThreadLocalImpl.h:
T& Local() override { return this->Internal.local(); }
where this->Internal is a tbb::enumerable_thread_specific<T>Internal;
jumping to tbb/enumerable_thread_specific.h, their local() implementation:
reference local(bool& exists) {
void* ptr = this->table_lookup(exists);
return *(T*)ptr;
}
past this there are two options, the first, which VTK will use by default (at least with the current TBB codebase) delegates to other implementations of thread-local functions, using either win32 functions or pthreads.
void* table_lookup( bool& exists ) {
void* found = get_tls();
if( found ) {
exists=true;
} else {
found = super::table_lookup(exists);
set_tls(found);
}
return found;
}
#if _WIN32||_WIN64
#if __TBB_WIN8UI_SUPPORT
using tls_key_t = DWORD;
void set_tls(void * value) { FlsSetValue(my_key, (LPVOID)value); }
void* get_tls() { return (void *)FlsGetValue(my_key); }
#else
using tls_key_t = DWORD;
void set_tls(void * value) { TlsSetValue(my_key, (LPVOID)value); }
void* get_tls() { return (void *)TlsGetValue(my_key); }
#endif
#else
using tls_key_t = pthread_key_t;
void set_tls( void * value ) const { pthread_setspecific(my_key, value); }
void* get_tls() const { return pthread_getspecific(my_key); }
#endif
It’s hard to tell exactly what the win32 APIs are actually doing, but their docs on TLSGetValue() say “TlsGetValue was implemented with speed as the primary goal” - so it probably doesn’t lock anything. Not sure what VTK’s stance on using direct OS calls (or win32 specifically), but perhaps doing something similar is an option?
They do have a non-delegated backend which is similar to what VTK is doing but uses std::thread::id directly for table lookups -
table_lookup() is implemented as
void* ets_base<ETS_key_type>::table_lookup( bool& exists ) {
const key_type k = ets_key_selector<ETS_key_type>::current_key();
__TBB_ASSERT(k != key_type(), nullptr);
void* found;
std::size_t h = std::hash<key_type>{}(k);
...
and the current_key() is derived from
template <std::size_t ThreadIDSize>
struct internal_ets_key_selector {
using key_type = std::thread::id;
static key_type current_key() {
return std::this_thread::get_id();
}
};