Operating System #27 Hardware Locks: Spinlock & its Usage

Operating System #27 Hardware Locks: Spinlock & its Usage
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High Level Constructs
• Spinlock
• Mutex
• Semaphore

15:40 Spinlocks Usage:
• One process will acquire the lock
• The other will wait in a loop repeatedly checking if the lock is available
• The lock becomes available when the former process releases it

Issues with Spinlocks
• No compiler optimizations should be allowed
– Should not make X a register variable
• Write the loop in assembly or use volatile
• Should not reorder memory loads and stores
• Use serialized instructions (which forces instructions not to be reordered)
• Luckly xchg is already implements serialization
• No caching of (X) possible. All xchg operations are bus transactions.
– CPU asserts the LOCK, to inform that there is a ‘locked ‘ memory access
• acquire function in spinlock invokes xchg in a loop…each operation is a bus transaction … huge performance hits.

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