diff --git a/src/hotspot/cpu/aarch64/c2_MacroAssembler_aarch64.cpp b/src/hotspot/cpu/aarch64/c2_MacroAssembler_aarch64.cpp
index 62831ee72ba05..b29be7213baf4 100644
--- a/src/hotspot/cpu/aarch64/c2_MacroAssembler_aarch64.cpp
+++ b/src/hotspot/cpu/aarch64/c2_MacroAssembler_aarch64.cpp
@@ -217,7 +217,7 @@ void C2_MacroAssembler::fast_unlock(Register objectReg, Register boxReg, Registe
// StoreLoad achieves this.
membar(StoreLoad);
- // Check if the entry lists are empty.
+ // Check if the entry lists are empty (EntryList first - by convention).
ldr(rscratch1, Address(tmp, ObjectMonitor::EntryList_offset()));
ldr(tmpReg, Address(tmp, ObjectMonitor::cxq_offset()));
orr(rscratch1, rscratch1, tmpReg);
@@ -538,7 +538,7 @@ void C2_MacroAssembler::fast_unlock_lightweight(Register obj, Register box, Regi
// StoreLoad achieves this.
membar(StoreLoad);
- // Check if the entry lists are empty.
+ // Check if the entry lists are empty (EntryList first - by convention).
ldr(rscratch1, Address(t1_monitor, ObjectMonitor::EntryList_offset()));
ldr(t3_t, Address(t1_monitor, ObjectMonitor::cxq_offset()));
orr(rscratch1, rscratch1, t3_t);
diff --git a/src/hotspot/cpu/ppc/macroAssembler_ppc.cpp b/src/hotspot/cpu/ppc/macroAssembler_ppc.cpp
index f036caa0675d2..9ed71f75c48ba 100644
--- a/src/hotspot/cpu/ppc/macroAssembler_ppc.cpp
+++ b/src/hotspot/cpu/ppc/macroAssembler_ppc.cpp
@@ -2736,7 +2736,7 @@ void MacroAssembler::compiler_fast_unlock_object(ConditionRegister flag, Registe
// StoreLoad achieves this.
membar(StoreLoad);
- // Check if the entry lists are empty.
+ // Check if the entry lists are empty (EntryList first - by convention).
ld(temp, in_bytes(ObjectMonitor::EntryList_offset()), current_header);
ld(displaced_header, in_bytes(ObjectMonitor::cxq_offset()), current_header);
orr(temp, temp, displaced_header); // Will be 0 if both are 0.
@@ -3083,7 +3083,7 @@ void MacroAssembler::compiler_fast_unlock_lightweight_object(ConditionRegister f
// StoreLoad achieves this.
membar(StoreLoad);
- // Check if the entry lists are empty.
+ // Check if the entry lists are empty (EntryList first - by convention).
ld(t, in_bytes(ObjectMonitor::EntryList_offset()), monitor);
ld(t2, in_bytes(ObjectMonitor::cxq_offset()), monitor);
orr(t, t, t2);
diff --git a/src/hotspot/cpu/riscv/c2_MacroAssembler_riscv.cpp b/src/hotspot/cpu/riscv/c2_MacroAssembler_riscv.cpp
index 75f87e35adf41..0ffdcbca72307 100644
--- a/src/hotspot/cpu/riscv/c2_MacroAssembler_riscv.cpp
+++ b/src/hotspot/cpu/riscv/c2_MacroAssembler_riscv.cpp
@@ -234,7 +234,7 @@ void C2_MacroAssembler::fast_unlock(Register objectReg, Register boxReg,
// StoreLoad achieves this.
membar(StoreLoad);
- // Check if the entry lists are empty.
+ // Check if the entry lists are empty (EntryList first - by convention).
ld(t0, Address(tmp, ObjectMonitor::EntryList_offset()));
ld(tmp1Reg, Address(tmp, ObjectMonitor::cxq_offset()));
orr(t0, t0, tmp1Reg);
@@ -566,7 +566,7 @@ void C2_MacroAssembler::fast_unlock_lightweight(Register obj, Register box,
// StoreLoad achieves this.
membar(StoreLoad);
- // Check if the entry lists are empty.
+ // Check if the entry lists are empty (EntryList first - by convention).
ld(t0, Address(tmp1_monitor, ObjectMonitor::EntryList_offset()));
ld(tmp3_t, Address(tmp1_monitor, ObjectMonitor::cxq_offset()));
orr(t0, t0, tmp3_t);
diff --git a/src/hotspot/cpu/s390/macroAssembler_s390.cpp b/src/hotspot/cpu/s390/macroAssembler_s390.cpp
index 6bfe5125959ad..d0537aef933e4 100644
--- a/src/hotspot/cpu/s390/macroAssembler_s390.cpp
+++ b/src/hotspot/cpu/s390/macroAssembler_s390.cpp
@@ -3667,7 +3667,7 @@ void MacroAssembler::compiler_fast_unlock_object(Register oop, Register box, Reg
// We need a full fence after clearing owner to avoid stranding.
z_fence();
- // Check if the entry lists are empty.
+ // Check if the entry lists are empty (EntryList first - by convention).
load_and_test_long(temp, Address(currentHeader, OM_OFFSET_NO_MONITOR_VALUE_TAG(EntryList)));
z_brne(check_succ);
load_and_test_long(temp, Address(currentHeader, OM_OFFSET_NO_MONITOR_VALUE_TAG(cxq)));
@@ -6510,7 +6510,7 @@ void MacroAssembler::compiler_fast_unlock_lightweight_object(Register obj, Regis
// We need a full fence after clearing owner to avoid stranding.
z_fence();
- // Check if the entry lists are empty.
+ // Check if the entry lists are empty (EntryList first - by convention).
load_and_test_long(tmp2, EntryList_address);
z_brne(check_succ);
load_and_test_long(tmp2, cxq_address);
diff --git a/src/hotspot/cpu/x86/c2_MacroAssembler_x86.cpp b/src/hotspot/cpu/x86/c2_MacroAssembler_x86.cpp
index 0eab2de2c64a4..026308736e76d 100644
--- a/src/hotspot/cpu/x86/c2_MacroAssembler_x86.cpp
+++ b/src/hotspot/cpu/x86/c2_MacroAssembler_x86.cpp
@@ -477,9 +477,9 @@ void C2_MacroAssembler::fast_unlock(Register objReg, Register boxReg, Register t
// StoreLoad achieves this.
membar(StoreLoad);
- // Check if the entry lists are empty.
- movptr(boxReg, Address(tmpReg, OM_OFFSET_NO_MONITOR_VALUE_TAG(cxq)));
- orptr(boxReg, Address(tmpReg, OM_OFFSET_NO_MONITOR_VALUE_TAG(EntryList)));
+ // Check if the entry lists are empty (EntryList first - by convention).
+ movptr(boxReg, Address(tmpReg, OM_OFFSET_NO_MONITOR_VALUE_TAG(EntryList)));
+ orptr(boxReg, Address(tmpReg, OM_OFFSET_NO_MONITOR_VALUE_TAG(cxq)));
jccb(Assembler::zero, LSuccess); // If so we are done.
// Check if there is a successor.
@@ -806,9 +806,9 @@ void C2_MacroAssembler::fast_unlock_lightweight(Register obj, Register reg_rax,
// StoreLoad achieves this.
membar(StoreLoad);
- // Check if the entry lists are empty.
- movptr(reg_rax, cxq_address);
- orptr(reg_rax, EntryList_address);
+ // Check if the entry lists are empty (EntryList first - by convention).
+ movptr(reg_rax, EntryList_address);
+ orptr(reg_rax, cxq_address);
jccb(Assembler::zero, unlocked); // If so we are done.
// Check if there is a successor.
diff --git a/src/hotspot/share/runtime/objectMonitor.cpp b/src/hotspot/share/runtime/objectMonitor.cpp
index 755d49d2c6c58..3ea987801db6a 100644
--- a/src/hotspot/share/runtime/objectMonitor.cpp
+++ b/src/hotspot/share/runtime/objectMonitor.cpp
@@ -1133,6 +1133,14 @@ void ObjectMonitor::UnlinkAfterAcquire(JavaThread* current, ObjectWaiter* curren
// or drain _cxq, we need to reacquire the lock before we can wake up
// (unpark) a waiting thread.
//
+// Note that we read the EntryList and then the cxq after dropping the
+// lock, so the values need not form a stable snapshot. In particular,
+// after reading the (empty) EntryList, another thread could acquire
+// and release the lock, moving any entries in the cxq to the
+// EntryList, causing the current thread to see an empty cxq and
+// conclude there are no waiters. But this is okay as the thread that
+// moved the cxq is responsible for waking the successor.
+//
// The CAS() in enter provides for safety and exclusion, while the
// MEMBAR in exit provides for progress and avoids stranding.
//