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Add StoreLoad barriers to ObjectMonitor::try_spin() for ARM64 Dekker … #50

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Add StoreLoad barriers to ObjectMonitor::try_spin() for ARM64 Dekker … #50

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12 changes: 12 additions & 0 deletions src/hotspot/share/runtime/objectMonitor.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2439,6 +2439,16 @@ bool ObjectMonitor::try_spin(JavaThread* current) {
if (!has_successor()) {
set_successor(current);
}
// Dekker/Lamport duality with exit(): ST _succ; MEMBAR; LD _owner.
// The exiter's side (release_clear_owner + storeload) is in exit().
// Here on the spinner's side, we need a StoreLoad barrier between
// setting _succ and reading _owner to prevent the CPU from reordering
// the _owner load before the _succ store. On ARM64 with MSVC
// /volatile:iso, Atomic::store/load are plain STR/LDR with no
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@swesonga swesonga Mar 10, 2026

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This comment should probably be reworded to remove the statement that /volatile:iso is in use if we intend to switch to /volatile:ms

// barrier, so without this fence the Dekker protocol is broken and
// the exiter may not see our successor designation while we may not
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@swesonga swesonga Mar 10, 2026

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Is this fence still needed now that Atomic::store/load are no longer plain STR/LDRs?

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@swesonga swesonga Mar 10, 2026

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Is this fence still needed now that Atomic::store/load are no longer plain STR/LDRs?

One reason this might not be needed is the explanation in https://github.com/openjdk/jdk/blob/9d4fbbe36d85d71ce850bb83bbfb1ce1d3e8dd23/src/hotspot/share/runtime/objectMonitor.cpp#L1586 - "the try_set_owner_from() below uses cmpxchg() so we get the fence down there." (this would be line 2492 in the right view of this file)

// see its lock release — leading to missed wakeups and starvation.
OrderAccess::storeload();
int64_t prv = NO_OWNER;

// There are three ways to exit the following loop:
Expand Down Expand Up @@ -2514,6 +2524,8 @@ bool ObjectMonitor::try_spin(JavaThread* current) {

if (!has_successor()) {
set_successor(current);
// See Dekker/storeload comment before the loop.
OrderAccess::storeload();
}
}

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