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In case a lock is contended it is better to do a load-and-test first before trying to get the lock with compare-and-swap. This helps to avoid unnecessary cache invalidations of the cacheline for the lock if the CPU has to wait for the lock. For an uncontended lock doing the compare-and-swap directly is a bit better, if the CPU does not have the cacheline in its cache yet the compare-and-swap will get it read-write immediately while a load-and-test would get it read-only first. Always to the load-and-test first to avoid the cacheline invalidations for the contended case outweight the potential read-only to read-write cacheline upgrade for the uncontended case. Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com> |
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