朴泊斋

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http://damocles.blogbus.com/logs/41378740.html

From ULK

In multiprocessor systems, semaphores are not always the best solution to the synchronization problems. Some kernel data structures should be protected from being concurrently accessed by kernel control paths that run on different CPUs. In this case, if the time required to update the data structure is short, a semaphore could be very inefficient. To check a semaphore, the kernel must insert a process in the semaphore list and then suspend it. Because both operations are relatively expensive, in the time it takes to complete them, the other kernel control path could have already released the semaphore. In these cases, multiprocessor operating systems use spin locks . A spin lock is very similar to a semaphore, but it has no process list; when a process finds the lock closed by another process, it "spins" around repeatedly, executing a tight instruction loop until the lock becomes open. Of course, spin locks are useless in a uniprocessor environment. When a kernel control path tries to access a locked data structure, it starts an endless loop. Therefore, the kernel control path that is updating the protected data structure would not have a chance to continue the execution and release the spin lock. The final result would be that the system hangs.