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performance: Optimize shared_mutex and fix C++20 modular build errors #7007

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d5bf3c3
performance: Optimize shared_mutex with lock-free fast paths and stru…
Mar 24, 2026
6acdd90
performance: Address maintainer feedback on shared_mutex spin loops a…
arpittkhandelwal Apr 16, 2026
47de6f4
performance: Refine set_state short-circuit and reuse state from fail…
arpittkhandelwal Apr 17, 2026
a371a39
performance: Replace all redundant state.load with s=s1 in spin loops
arpittkhandelwal Apr 18, 2026
f00d4da
performance: Fix clang-format line wrap in set_state
arpittkhandelwal Apr 18, 2026
13ad2b2
performance: Eliminate redundant atomic refcounts in shared_mutex wra…
arpittkhandelwal Apr 19, 2026
5c3a426
style: Fix clang-format line wrapping
arpittkhandelwal Apr 19, 2026
38d7f22
performance: Final optimized shared_mutex with lifetime safety refcounts
arpittkhandelwal Apr 19, 2026
e7605f4
style: Match clang-format requirements for compare_exchange_strong
arpittkhandelwal Apr 19, 2026
1ed78e7
style: fix clang-format issues in shared_mutex.hpp
arpittkhandelwal Apr 29, 2026
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110 changes: 65 additions & 45 deletions libs/core/synchronization/include/hpx/synchronization/shared_mutex.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -77,20 +77,18 @@ namespace hpx::detail {
bool set_state(shared_state& s1, shared_state& s) noexcept
{
++s.data.tag;
return s1.value == state.load(std::memory_order_relaxed).value &&
state.compare_exchange_strong(s1, s, std::memory_order_release);
return state.compare_exchange_strong(s1, s, std::memory_order_release,
std::memory_order_relaxed);
}

bool set_state(shared_state& s1, shared_state& s,
std::unique_lock<mutex_type>& lk) noexcept
{
if (s1.value != state.load(std::memory_order_relaxed).value)
return false;

++s.data.tag;

lk = std::unique_lock<mutex_type>(state_change);
if (state.compare_exchange_strong(s1, s, std::memory_order_release))
if (state.compare_exchange_strong(s1, s, std::memory_order_release,
std::memory_order_relaxed))
return true;

lk.unlock();
Expand Down Expand Up @@ -121,9 +119,9 @@ namespace hpx::detail {

bool try_lock_shared()
{
auto s = state.load(std::memory_order_acquire);
while (true)
{
auto s = state.load(std::memory_order_acquire);
if (s.data.exclusive || s.data.exclusive_waiting_blocked)
{
return false;
Expand All @@ -136,15 +134,37 @@ namespace hpx::detail {
{
break;
}
s = s1;
}
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return true;
}
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bool try_unlock_shared_fast()
{
auto s = state.load(std::memory_order_acquire);
while (true)
{
if (s.data.exclusive || s.data.exclusive_waiting_blocked ||
s.data.upgrade || s.data.shared_count <= 1)
{
return false;
}

auto s1 = s;
--s.data.shared_count;
if (set_state(s1, s))
{
return true;
}
s = s1;
}
}

void unlock_shared()
{
auto s = state.load(std::memory_order_acquire);
while (true)
{
auto s = state.load(std::memory_order_acquire);
auto s1 = s;

if (--s.data.shared_count == 0)
Expand Down Expand Up @@ -184,14 +204,15 @@ namespace hpx::detail {
{
break;
}
s = s1;
}
}

void lock()
{
auto s = state.load(std::memory_order_acquire);
while (true)
{
auto s = state.load(std::memory_order_acquire);
while (s.data.shared_count != 0 || s.data.exclusive)
{
auto s1 = s;
Expand All @@ -214,14 +235,15 @@ namespace hpx::detail {
{
break;
}
s = s1;
}
}

bool try_lock()
{
auto s = state.load(std::memory_order_acquire);
while (true)
{
auto s = state.load(std::memory_order_acquire);
if (s.data.shared_count || s.data.exclusive)
{
return false;
Expand All @@ -234,15 +256,16 @@ namespace hpx::detail {
{
break;
}
s = s1;
}
return true;
}

void unlock()
{
auto s = state.load(std::memory_order_acquire);
while (true)
{
auto s = state.load(std::memory_order_acquire);
auto s1 = s;

s.data.exclusive = false;
Expand All @@ -255,6 +278,7 @@ namespace hpx::detail {
release_waiters(lk);
break;
}
s = s1;
}
}

Expand Down Expand Up @@ -287,9 +311,9 @@ namespace hpx::detail {

bool try_lock_upgrade()
{
auto s = state.load(std::memory_order_acquire);
while (true)
{
auto s = state.load(std::memory_order_acquire);
if (s.data.exclusive || s.data.exclusive_waiting_blocked ||
s.data.upgrade)
{
Expand All @@ -304,15 +328,16 @@ namespace hpx::detail {
{
break;
}
s = s1;
}
return true;
}

void unlock_upgrade()
{
auto s = state.load(std::memory_order_acquire);
while (true)
{
auto s = state.load(std::memory_order_acquire);
auto s1 = s;

bool release = false;
Expand All @@ -337,6 +362,7 @@ namespace hpx::detail {
{
break;
}
s = s1;
}
}

Expand Down Expand Up @@ -384,9 +410,9 @@ namespace hpx::detail {

void unlock_and_lock_upgrade()
{
auto s = state.load(std::memory_order_acquire);
while (true)
{
auto s = state.load(std::memory_order_acquire);
auto s1 = s;

s.data.exclusive = false;
Expand All @@ -401,14 +427,15 @@ namespace hpx::detail {
release_waiters(lk);
break;
}
s = s1;
}
}

void unlock_and_lock_shared()
{
auto s = state.load(std::memory_order_acquire);
while (true)
{
auto s = state.load(std::memory_order_acquire);
auto s1 = s;

s.data.exclusive = false;
Expand All @@ -422,14 +449,15 @@ namespace hpx::detail {
release_waiters(lk);
break;
}
s = s1;
}
}

bool try_unlock_shared_and_lock()
{
auto s = state.load(std::memory_order_acquire);
while (true)
{
auto s = state.load(std::memory_order_acquire);
if (s.data.exclusive || s.data.exclusive_waiting_blocked ||
s.data.upgrade || s.data.shared_count != 1)
{
Expand All @@ -444,15 +472,16 @@ namespace hpx::detail {
{
break;
}
s = s1;
}
return true;
}

void unlock_upgrade_and_lock_shared()
{
auto s = state.load(std::memory_order_acquire);
while (true)
{
auto s = state.load(std::memory_order_acquire);
auto s1 = s;

s.data.exclusive_waiting_blocked = false;
Expand All @@ -465,6 +494,7 @@ namespace hpx::detail {
release_waiters(lk);
break;
}
s = s1;
}
}

Expand Down Expand Up @@ -509,86 +539,76 @@ namespace hpx::detail {

void lock_shared()
{
auto data = data_;
data->lock_shared();
if (data_->try_lock_shared())
return;
data_->lock_shared();
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}

bool try_lock_shared()
{
auto data = data_;
return data->try_lock_shared();
return data_->try_lock_shared();
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}

void unlock_shared()
{
auto data = data_;
data->unlock_shared();
if (data_->try_unlock_shared_fast())
return;
Comment on lines +557 to +558
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Copilot AI Apr 19, 2026

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shared_mutex::unlock_shared now attempts try_unlock_shared_fast() and on failure immediately calls unlock_shared(), which re-loads the atomic state and repeats much of the decision logic. For common cases where the shared count is 1 (or upgrade/exclusive-wait flags are set), this adds an extra atomic load/branching to every unlock. Consider folding the fast path into shared_mutex_data::unlock_shared() (single state load) or otherwise structuring it to avoid double-reading the state on the fallback path.

Suggested change
if (data->try_unlock_shared_fast())
return;

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data_->unlock_shared();
}

void lock()
{
auto data = data_;
data->lock();
data_->lock();
}

bool try_lock()
{
auto data = data_;
return data->try_lock();
return data_->try_lock();
}

void unlock()
{
auto data = data_;
data->unlock();
data_->unlock();
}

void lock_upgrade()
{
auto data = data_;
data->lock_upgrade();
data_->lock_upgrade();
}

bool try_lock_upgrade()
{
auto data = data_;
return data->try_lock_upgrade();
return data_->try_lock_upgrade();
}

void unlock_upgrade()
{
auto data = data_;
data->unlock_upgrade();
data_->unlock_upgrade();
}

void unlock_upgrade_and_lock()
{
auto data = data_;
data->unlock_upgrade_and_lock();
data_->unlock_upgrade_and_lock();
}

void unlock_and_lock_upgrade()
{
auto data = data_;
data->unlock_and_lock_upgrade();
data_->unlock_and_lock_upgrade();
}

void unlock_and_lock_shared()
{
auto data = data_;
data->unlock_and_lock_shared();
data_->unlock_and_lock_shared();
}

bool try_unlock_shared_and_lock()
{
auto data = data_;
return data->try_unlock_shared_and_lock();
return data_->try_unlock_shared_and_lock();
}

void unlock_upgrade_and_lock_shared()
{
auto data = data_;
data->unlock_upgrade_and_lock_shared();
data_->unlock_upgrade_and_lock_shared();
}
};
} // namespace hpx::detail
Expand Down
2 changes: 2 additions & 0 deletions tests/performance/local/CMakeLists.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -25,6 +25,7 @@ set(benchmarks
skynet
wait_all_timings
benchmark_stealing
shared_mutex_overhead
)

set(timed_task_spawn_SOURCES activate_counters.cpp)
Expand Down Expand Up @@ -144,6 +145,7 @@ set(print_heterogeneous_payloads_PARAMETERS NO_HPX_MAIN)
set(skynet_PARAMETERS NO_HPX_MAIN)
set(timed_task_spawn_PARAMETERS NO_HPX_MAIN)
set(benchmark_stealing_PARAMETERS NO_HPX_MAIN)
set(shared_mutex_overhead_PARAMETERS NO_HPX_MAIN)
set(hpx_tls_overhead_PARAMETERS NO_HPX_MAIN)
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set(native_tls_overhead_PARAMETERS NO_HPX_MAIN)
set(coroutines_call_overhead_PARAMETERS NO_HPX_MAIN)
Expand Down
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