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Consider two threads that are executing some statements:
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Thread 1
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Statement 1
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(statement does not use a volatile variable)
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Statement 2
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(statement does not use a volatile variable)
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Statement 3
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(statement is a write to a volatile variable v)
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Thread 2
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Statement 4
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Thread 1 and Thread 2 have a happens-before relationship such that Thread 2 does not start before Thread 1 finishes. This is established by the semantics of volatile accesses. Sequential consistency of volatile accesses provides certain visibility and reordering guarantees:
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